To remove a user from your users list, click the red "X" which appears next to each user.  If you don't see this red "X", then you aren't logged in as an Administrator in the PBworks service.

When a user is removed, the space does not appear when the user is logged in to My PBworks.  If the Workspace Security setting of the space is set to only allow users you invite or approve, then the user will lose access to the space. 

If the Workspace Security is set to allow anyone to view the space, then the user will still be able to view the space, but not make any comments or edits.