Suggestion:  Allow user to set the price for anonymous messages.  

Example - John Doe hates having to put up with the bullshit anonymous messages he gets from time to time.  1 Credit isn't enough to put up with that anymore... but maybe 5 credits would be?

John Doe goes into his general options and changes his "Anonymous Message Fee" from 1 credit minimum (default) to 5 credits minimum.  Now, if someone wants to spam him with random hate mail, he'll at least get 5 credits per message rather than the standard 1.

Alternate Suggestion:  A check box which allows you to block anonymous credit donations.

I can't see many people turning down free credits.

this doesnt seem feasible, side note if anyone is spamming messages with credits have at er

I can't see many people turning down free credits.

Probably not - but I can say there have been times I've thought "you know, you're not paying me enough to put up with your b.s./cowardly messages which I can't respond to." and at those times, had I had the option, I would have turned them off.