While I am sure that this happens, in general the guest voting patterns do resemble member voting patterns. It does depends though, if the poll is about something that people feel strongly about, or if there is something to gain/loose, then guest voting is not useful.
For example:
Which do you prefer:
1. pie
2. cake
3. ice cream
Such poll will generally not attract abusive voting
Does god exist:
1 yes
2. no
Such poll will attract abusive voting.
In any case, what was happening in the situation described above is that spam bots where not voting more for specific answers, but were voting equally on all answers. For example on one poll there were 6 answers and the bot added 1950 votes to every answer.