1) I would definitely implement a cap here - but something reasonably high... maybe 2GB is free, pay for more? 2) An app I wrote allowed this. We put a limit i

1) I would definitely implement a cap here - but something reasonably high... maybe 2GB is free, pay for more? 2) An app I wrote allowed this. We put a limit in that prevented more than 4 changes to occur within 10 minutes. If the user attempted a 5th change, they would get a message asking them to wait a few minutes. 3) Implementing a public/private key here is relatively trivial - any reason not to do it? 4) Hardware firewalls will help with a large portion of this. I had a server that would get brought to it's knees about twice a week from that fracking SQL worm. Added a hardware firewall instead of relying on the "non-standard" port and hard-to-remember password, and problem pretty much solved. For applications that I write or control, I would put reasonable effort into "shaping" the worst possible abuses. For example, point #2 above - there's no real reason the user will ever change their plan more than a few times in a few minutes, so introduce a delay once a certain limit is reached. It won't cause any problems and will prevent confusion at the administration side.