BigBomb,
Just backup, then delete the contents of the .htpasswd file.
When your members try to log in, they will get an Apache 401 error. You can use .htaccess to point the error to an .html page you make informing your members their login is no longer valid, why it isn't, and how to sign up for your new processor.
Make sure they know how to cancel charges from Paymonde, as well. Let them know they will keep getting charged unless they cancel membership themselves.
Did a quick search on Google and here is a page to help you redirect the Apache 401 error code:
http://www.indiana.edu/~wmhome/tool_...errordoc.shtml