There is no way to "disable a database user" per se. The way i do this is to remap the User i want to "disable" to a different Login, a temporary one that no one could ever usemto connect to the instance. Then when i want to "enable" the User again, i remap it to the original Login thereby reinstating access.
There are no special teachers of virtue, because virtue is taught by the whole community.
--Plato