If you wish to remove the advertisement altogeather
Open SMS or SCCM Administrator Console - Expand Advertisements - Find the Advertisement and delete it
If you wish to remove it from a particular machine
Open SMS or SCCM Administrator Console - Expand Advertisements - Find the Advertisement and find the associated collection.
Remove the machine/user from the collection once Systems Management Policy for Computers/Users is updated the advertisement will disappear.
Cheers
