There is no functionality in Snitz that allows you to enable 'guest posting', at least not in the default package. There are some (unreleased) mods out there that enable you do to do so, however they'll require intensive modifications to the base code.
For example for my own board I've done it by creating a guest account, and modified the post page to show the guest account details. To prevent abuse of the guest account, I've disabled logging in using the guest account, and I've disabled all delete/edit/profile pages for the guest account.
This is not going to accomplish what you want. If people aren't interested enough to participate now, allowing any spammer to post isn't going to change their minds.
I manually keep my forum clean of spam at this point, so there will be nothing like that to annoy my users. I just wanted a way to make it easy for people to post in order to spur growth.