Click on the
Database Manager link 
in the Site
Management area.
Click on the delete icon 
next to the specific user underneath the name of the database in the Databases area.
A user will no longer be able to access a particular database when you have removed their permissions to that database.
To remove a user's permissions:
Click on the
Database Manager link in the Site
Management area.
Click on the delete icon
next to the specific user underneath the name of the database in the Databases area.
