Single pay access is not enough imo, bitcoin spam preventions is transaction fees, not only that but very important is that it's exponential, not linear or fixed value, relays could request a payment to whitelist and then if you start spamming demand another payment and that would increase if you continue. Maybe the other way around could also work, you would start paying a monthly fee and if you behaved nicely over some tine you would start paying less monthly.
Discussion
subscription models are simple... you pay a rate at which the company makes a margin on average use, if your use goes above that rate, they move you to a rate limited service priority until your billing cycle resumes, or you upgrade to a higher service level
"nice" should have nothing to do with it, you pay, you receive. no pay, no receive.