This is exactly what is happening. Because the /r command is unreliable, I use the Alt+R combination to set my tell target. It cycles through those who have sent you tells recently starting with the most recent. If I've been spammed, then I have to cycle past the gil spammer to get to the person I want to talk to.

The game probably should filter against the blacklist for commands like /r, and the Alt+R combination. The client blocks the tell, so it seems reasonable that it should also know you don't want to reply to it.