Sure, but ReplyGuy costs to spam are basically zero currently, and what income are these replies producing? At some level of difficulty, there is negative benefit to spamming this way. Relays can apply difficulty requirements for new npubs and existing, and keep raising the difficulty if needed
Discussion
I like this idea. Huge PoW to register on a relay might make using disposable npubs useless (too expensive) for an adversary (spammer)