You can easily revoke permissions given to a user, as follows:
- Browse to the Edit Permissions page for the permission scheme you wish to configure
- Click on the Remove link for the permission you wish to revoke, and click on Remove again
When you are trying to revoke permissions to prevent users from gaining access to certain things, you need to make sure no other user options are granted the same permission that might be applied to the same user. For example, if you have both the Single User and Group options set for the Browse Projects permission, then you will need to make sure that you revoke the Single User option and also make sure that the user does not belong to the Group option selected, so you do not have a loophole in your security settings.