spam-whacking: how much help is banning by ip?

[goddess_dix]

Was hoping for some sugestions from more experienced forum admins; I recently took over a reasonably-sized board (abt 11,000 members) and am still learning the ropes.

When we have spam registrations, currently my mods ban by user, email and ip. Looking into available mods to make this process easier for them, I saw mention that banning the email addy wasn't needed if you're retaining the banned users with unique emails required. This makes sense to me. (although i'm not sure this is necessarily the best approach, either.)

And I started wondering about banning by ip: how helpful is it? do most of the spammers use proxys anyway? is it very likely that legitimate users will get caught in the net if you use full ip's?

and over time, will all these bans set up slow down the board?

If I can clearly understand what most needs to be done, then it will be easier for me to select among the available hacks and options to give my mods the best, easiest to work tools i can find. that's my goal.

your thoughts appreciated.