Spambot Prevention?

Are there any measures that can be taken to prevent spambots from creating user accounts? Any chance of making new users decipher a sequence of partially distorted letters/numbers, as part of their account creation? (I presume this feature is indeed a 'spambot filter')

I find their 'thread hijacking' antics (in particular) slightly irksome.

Cheers.

[img]http://t3.gstatic.com/images?q=tbn:ANd9GcTKSrxOuY6grhSLxLLvq40GBzilMXkZfIHgbo7YMRSq6bdrcARl[/img]