I am unsure what you want. Your last two posts say to entirely different things.
1. If you want to remove a users sig: ACP --> Users and Groups --> Find the user --> Select Form = Signature --> Delete or edit as you like
2. If you want to remove a user's ability to have a sig: ACP --> Users and Groups --> Users' Permissions --> Find the user --> Advanced Permissions --> "Can use signature" = never.
If you expect to have to do this for more than a few users, it is best to create a "no sig" group and set the group's sig permissions to "never" instead.
3. If you want to remove/delete a user completely: ACP --> Users and Groups --> Find the user --> Delete User (with "retain posts").