Automatic Payment Proposal

End User has created a proposal for 9/19/2007 with a specific "payment method". On the proposal list, there are a number of customers that should not have been included in the proposal because there master data did not have the specific "payment method" given. Upon further inquiry we found out that a "payment method" was accidently alloted to a number of customers by another users while entering manual transactions. Is there a way to remove that "payment method" in a mass removal, or does it have to be done one by one. Please help. very urgent