What kind of value would having a forum user's email address give you? Unless by "monetize" you just mean "send them activity notices so they'll come back", it sounds like you want to spam them.
Personally, I recommend monetizing internet forums by selling people things and having them buy them for money. You can keep interest by just being interesting.
I don't think sending your users emails should be considered spam. Mailing lists and email upsells can be very lucrative, even if you're not emailing users immediately, an email address is a valuable thing to be leaving on the table.
Personally, I recommend monetizing internet forums by selling people things and having them buy them for money. You can keep interest by just being interesting.