At this time there is only a way to prevent a user from receiving them, not sending them out. If you go into your user CP you will find the option to Ignore PT Box Cleanup messages, but each user will have to do that on an individual basis.
"I have moderated and managed discussion boards since 1999 for various sites and networks, including a message board with millions of viewers for a top 25 internet company. FusionBB is shaping up to be one of the most feature rich and flexable message board programs available on the market. I am very excited to see the future of FusionBB, and convert over my two forum sites to FusionBB when available to the public!"