I have yet to find a way to delete/change the one's from Gabby & Ty, however there are others from random NPC's in buildings where you can get out or change rooms and come back in and they'll ask you the same thing again, so you can get a chance to change those, at least.
Mh.. ye I knew about the NPCs in the buildings. I hope there is a way to change the other ones (if there are any more situations like with Gabby & Ty), I would find it annoying if I don't know how the game puts my text in and then something like "textextext!." happens...