attachments and bots - can we block bots?

[Phantasmagoric]

I've read a fair few threads here concerning bots and membership groups, even though it is default in phpBB to have a bot usergroup I keep seeing it stated here that this is called cloaking and get's us banned by search engines etcetera.

I've just had google spider access a Skin download attachment on my site, I did a bit of searching on the net and it seems some bots can get real nasty doing this, on occasion using so much bandwidth that sites get closed by servers.

As I am providing access to some Skins on my site to guests I need to leave the attachment access to guests open, but in doing so I am opening my site up to possible attacks by bad bots. Is there anything we can do to help protect our sites from this sort of behaviour without punishing genuine human guests in the process?