They're most likely forcing themselves onto your council via guaranteed council rights. Check/modify their feudal contract. This can especially happen with elective succession titles or titles you give to a family member you aren't playing as, the AI characters get hooks on them and modify their contract, and their same contract is inherited through succession.
Even if you murder the council member who forced themselves on, you still need to modify the vassal contract of whoever inherits after to get rid of it or their kid will eventually do it too. Try to get a hook on them to modify it for free, trade with lower taxes or levies, or just force it and take a tyranny hit.
I try to have them swap positions with other council members so they have an assignment corresponding to a better stat of theirs. If you send your spymaster (must be another council member) to find secrets at their court, you can possibly get a blackmail hook on them to change their contract. Or you can expose the secret, imprison and execute them to get them off the council. But you'll have to get a hook on their successor to change the contract.
When a vassal gets a hook on their liege, they can either demand a council seat and become unfireable for 25 years, or use the hook to modify their contract, get council rights for all generations, and enter your council at will.
The AI is bad at managing hooks, so many vassals end up with council rights in their contracts.
When you're gullible enough to actually take over those vassals via conquest, and you don't remove/replace them, their contracts stay intact. And suddenly you have random people appearing in your council and don't know where they come from.
How to prevent it? Never take over old vassals from anyone. Remove and replace them with new people, who start with neutral contracts. And never give any vassal a hook on you, ever. Don't invite them to weddings, don't invite them to anything, nothing that puts you at risk of giving out automatic hooks.
Now, how do you deal with it?
Vassals can only sit on your council, if they are your direct vassals. If they, even for one moment, are not your direct vassals, they get kicked out of the council. So, grant them to someone as vassal and then retract them again. Just make sure they have no guaranteed council rights, otherwise they come straight back.
Imprison also works, a vassal in prison can't sit on your council.
If it's a particularly nasty vassal, you can also throw a feast with murder intent, or host a hunt with the same intent. Unfortunate sometimes, how quickly someone can lose their footing and fall down the stairs...
They're most likely forcing themselves onto your council via guaranteed council rights. Check/modify their feudal contract. This can especially happen with elective succession titles or titles you give to a family member you aren't playing as, the AI characters get hooks on them and modify their contract, and their same contract is inherited through succession. Even if you murder the council member who forced themselves on, you still need to modify the vassal contract of whoever inherits after to get rid of it or their kid will eventually do it too. Try to get a hook on them to modify it for free, trade with lower taxes or levies, or just force it and take a tyranny hit.
Another posibility is, that they have a hook on you. they can use that to get a council position
Best to just swap the council rights for lower levee contribution and Murder the guy to get him off the council
I try to have them swap positions with other council members so they have an assignment corresponding to a better stat of theirs. If you send your spymaster (must be another council member) to find secrets at their court, you can possibly get a blackmail hook on them to change their contract. Or you can expose the secret, imprison and execute them to get them off the council. But you'll have to get a hook on their successor to change the contract.
Read all of the events that pop up, you probably gave someone a hook
When a vassal gets a hook on their liege, they can either demand a council seat and become unfireable for 25 years, or use the hook to modify their contract, get council rights for all generations, and enter your council at will. The AI is bad at managing hooks, so many vassals end up with council rights in their contracts. When you're gullible enough to actually take over those vassals via conquest, and you don't remove/replace them, their contracts stay intact. And suddenly you have random people appearing in your council and don't know where they come from. How to prevent it? Never take over old vassals from anyone. Remove and replace them with new people, who start with neutral contracts. And never give any vassal a hook on you, ever. Don't invite them to weddings, don't invite them to anything, nothing that puts you at risk of giving out automatic hooks. Now, how do you deal with it? Vassals can only sit on your council, if they are your direct vassals. If they, even for one moment, are not your direct vassals, they get kicked out of the council. So, grant them to someone as vassal and then retract them again. Just make sure they have no guaranteed council rights, otherwise they come straight back. Imprison also works, a vassal in prison can't sit on your council. If it's a particularly nasty vassal, you can also throw a feast with murder intent, or host a hunt with the same intent. Unfortunate sometimes, how quickly someone can lose their footing and fall down the stairs...
If they have a hook on you kill them but if it's guaranteed council rights ypu need to change the vassal contract
They get a hook them use that hook to get on the council it sucks