This script should work just fine:
You may want to modify it a little so it first checks if the user exists:
drop Login [login_name]
go
sp_msforeachDB 'Use [?]; DROP USER [user_name];'
Ysaias Portes | Forward Thinkers Consulting
sp_SQLFlightRecorder — open-source SQL Server diagnostic recorder | forwardthinkersconsulting.com