The first thing I would try is adding a autocomplete="off" attribute to the <input type="password" /> field.  For Web Access, this is on line 78 of Login.aspx.  Unfortunately, it seems like browsers are moving away from following this: http://stackoverflow.com/questions/3868299/is-autocomplete-off-compatible-with-all-modern-browsers/21348793#21348793.  I think their position is that the user should be the one who decides if remembering a password is appropriate.  You might also investigate enforcing this with group policy.