Hey Dom and welcome to the forums!
That’s a great question. I wish I could say that is possible, but I’m afraid it isn’t right out of the box. You can refine the results returned from an iCal feed in the iCal Importer settings (Events > Import > iCal) for certain keywords, but to include those events rather than exclude them.
I think that would make an excellent feature, though, and would love for you to add it to our feature request forum, if you are willing to. I could see that being handy for other people as well and would be interested in seeing it collect votes.
Sorry for the bad news, but does this at least help answer your question? Please let me know.
Thanks,
Geoff