Just a reminder, which I'll pin here, that we do have rules and guidelines in place -- the link is always available under the banner. It is handily labelled 'Rules and Guidelines.'
To make this very clear, rule number 4, which includes 'impersonating other members of the forum', will be applied if anyone attempts to login as someone else, by guessing a password. Invalid login attempts are logged, along with IP numbers, so it is normally easy to track down the offender.
My advice -- don't do it, not even in jest. Personally I think that the only reason anyone would want to do this is to undermine the forum integrity, which is a very serious matter. I will report any attempt to the offender's ISP.
-Robin Corn