For paid relays, anyone can read from them, but only paid users can write to them.

nos.lol is a an older and well known public relay and does have spam, but nostr:nprofile1qqsflmrj64um42nh9tu7w8nr3dffy9tjrt8xururype9ajle7alctvgppemhxue69uhkummn9ekx7mp0qyg8wumn8ghj7mn0wd68ytnddakj7qg4waehxw309ahx7um5wghx77r5wghxgetk9uukdx2g is now using AI to combat spam and he's doing a great job with it.