SPAM.. the annoying nature of the beast.. out filter here at work compiles the weekly statistics.. we average around 56% of inbound mail that is spam and blocked. Less than .005 % of those are false positives.
That is the reality of what is out on the net.. and about the only reason I would ever consider supporting a fee on email.. to cut down the spammer/blast mailings.. but that really would be just off setting one cost (spam filtering) with another cost in terms of a bill for email traffic at best.
There isn't a good way of controlling it without being intrusive to the members or the admin.. whether it be an entry question to post or join, an approval process for the first x posts, a restriction on the type of posts (no links) for the first x amount of posts. Etc.
No matter what method, there will be some net affect to either the members or the admin. My opinion anyway.. nature of a forum.
"The oranges are dry; the apples are mealy; and the papayas... I don't know what's going on with the papayas!"