Hello,
Shortly will describe 3 ideas:
1) Make daily server reboots like 2-3 times a week. It's takes about 30 min, but it will be much less lag in gameplay and ppl will enjoy it - belive me. Example of a good daily reboot MMO - EVE online (were lags still make ppl angry).
2) Accounts and characters creation limits. For example: you can create 1 character on newly created accout once in a 24h. Something similar with account creation.
3) Chat characters leight limits. Actually gess it will be hard to make. A good example could be warcraft 3 private servers chat scripts, were you can't spam like an idiot, but still it's okay for fast chating with a friends.
That is all for now, tnx for attention and sorry for my bad english.
Best regards,
Maslow