You can but that doesn't help you keep the flood of contributions out when you don't have the time or resources to properly discern good from bad. Maintainers would rather have 10 good human authored patches than 100 patches from LLMs, even if 20 of them are good. Even if 50 of them are good, probably.
As if a rule against LLMs actually stops those sorts of spam contributions.
The only thing it does is filter good contributors out, while you still have to deal with the bad ones.
It makes it easier to filter. Most LLM spam can be easily noticed. And those that aren't automatically filtered, can fairly easily be closed by the maintainer - when they don't have the weight to assess each on their validity.