On phpBB the first timezone in the drop down list is UTC-12 which conveniently is uninhabitable. The default board timezone is pre-selected for new registrants. Many bots will submit -12, a little bit of modification stops them.
I use layers of tests, you just have to identify what you can do. You might be able to incorporate that into any registration where the first selection in a drop down list no one would pick and pre-select something in the middle of the list.