While traditional UCE (get rich quick, enlarge your body parts, Nigerian barristers) are handled adequately, I'm still receiving a lot of not quite unsolicited spam. This is typically from commercial services forwarding "invites" from my "friends" and then "reminding" me of their services. Typical offenders are Facebook, Linkedin, dropbox, bebox, etc etc.

(Of course, none of these services provide any way of opting out, except possibly by registering, which they will then take as an invitation to stuff your mailbox with even more crap)

What is a good way to deal with these? I can of course junk them using procmail, but is it a better idea to e.g. bounce them, or at least send a reply informing the sender (and "friend") that I am not interested in their service nor their spam. Any solutions to this?