Yes it is true that any registered user may send an email to any other registered user of that Nabble forum, by visiting that user's Profile screen, but I don't think this is relevant here as the OP was talking about posts to his forum, not private messages.
Because Nabble has no internal PM system email is the only way for a Nabble user to contact another privately is by email.
If you are receiving junk emails to your administrator address and they appear to be generated by a user then the most likely cause is that that user's has your address in his address book and it has been hacked and the spam is not directly related to Nabble's system.
I say that on the basis that I have seen no other reports of mass abuse of Nabble's email system, so you appear to be suffering an isolated case.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.