I think JacksiroKe's Advanced Blocker Plugin might be useful for that; it says:
Blocked user can not view profile, wall or account of the person that blocked him/her
so blocked users can't send private messages to the one who blocked him.
esqeudero's PM Plus also has a similar feature:
Adds blocking/unblocking users from sending PM
These plugins also include other features that might or might not be useful; in any way, you can contact them to check how they work. They might be available for customization as well.
Unfortunately, I can't vouch for them; does somebody know if they're good for
this purpose? Your opinion is really important and might help lipsmega (and
other users as well) make a good decision.
Here are other suggestions:
- If the project maintainer agrees with you on this topic, that feature might eventually be included in Q2A itself
- Other community members have also worked on other anti-spam tools (see the Add-ons page), maybe they can give you a hand
- Ask one of the service providers to help you on that (or send me a private message)
If you find a solution, please help other community members by sharing what it was.
I hope you find it useful. Have a nice day.