Because Islam does not give freedom of thought, speech, or action, and because it tells Muslims to lie, cheat, loot, rape the Kafirs, no Muslim can rationally debate with kafirs.
Therefore, the best way to deal with the Muslims on forums is to not talk with them at all, and send their e-mails to junk mail folder or delete them.
Notice that while we kafirs hardly go to a Muslim forum/group, the Muslims, in contrast, purposely invade into kafir forums and groups.
And we can stop them wasting our time.
The kafirs need to focus effort to declare Islam illegal in their country.