The Spamfire Revenge menu lets you send a bounce message back to the spammer who sent you a spam. Some spammers may take this to mean that your email address is invalid, and they might remove you from their list. This might reduce the amount of spam you receive.
Keep in mind, though, that many spammers use bogus email addresses and almost all spammers ignore bounces. Bouncing might help a little, but don't expect a huge reduction in your spam traffic.
When bouncing is worthwhile
There are some messages you should bounce if you spot them. You may receive probe messages, which contain nothing but gibberish or which contain completely irrelevant nonsense information (like a note from an unknown person thanking you for lunch).
These may be messages that a spammer is sending to randomly chosen addresses in an attempt to locate new email addresses. In this case, the spammer is actively monitoring the bounces. If no bounce is received, your email address is presumed valid, and you will start receiving more spam. If you bounce these messages, the spammer will see it and may not add your address to their list.