Is there a way to sign out some specific user (not current one) from the episerver?
Out of the box there is no such functionality. To "logout" some user, first depends what authentication method are you using to login users (and then this is not CMS specific issue but general .NET5/6 ASP.NET thing).
But lets say you are using OIDC, so the authentication/authorization is eventually stored to cookie to the client. So you don't know anything who is "logged in" in the system, unless you build some custom implementatio to track this and then you could have some code to "revoke" login, but you would need to be able to also revoke any refresh tokens etc, so it is not that simple.