Regarding bans it all happens on a personal level. If the reviewer visits a flagged account that person decides how far to go. Some just terminate one account with all tumblrs on it, others use the tools at their disposal to find more accounts, i.e. by checking for similar accounts. One of the patterns they look for are common links pointing to the same servers. Other tools include checking for same IP ranges that were used within certain time periods. And of course they have a partial fingerprint of your system, etc. It really is a very easy task for them if they want to go after spammers. They don't use automated ways to suspend accounts. They are just used for flagging, but deletion takes place by a human reviewer.