Trial accounts are already blocked from most chat (as well as a lot of other features of the game).

And disabling new players only hurts new players and therefore the game itself. It makes no difference to RMT bots what level they need to be. They're scripted to auto-level to wherever they need to be. It would literally be just a matter of copying a piece of the gathering bots' script and pasting it into the spam bots' script. Then the spam bots are all level 30, and the only below-level-30 characters being impacted by the rule are genuine new players, i.e. the worst possible place to be putting chat restrictions.

We've already got the feature to get rid of spam bots. Report and block them whenever you see one.