Windows: Pidgin may be our pick for best instant messaging application in Windows, but one thing it can't do is block out unwanted IMs or buddy requests. That's where Pidgin Privacy Please comes in.
Pidgin Privacy Please is a free add-on for Pidgin that stops IM spam in its tracks. It fools spammers who IM you by auto-responding as though you're offline, or can be configured with an auto-response message that you choose. You can even add a kind of captcha to your auto-responses, where the person has to prove they're human before the IM will reach you.
The add-on can also be configured to block messages from people not on your contact list entirely, or block messages that match a regular expression that you provide. It can even suppress repeated buddy authorisation requests. Pidgin Privacy Please is free and works where Pidgin does.