Yes, you can do this. It only applies to new traffic coming in after the filter was added, though. First thing to do though would be to make a separate profile for it, because you risk messing up your data if it's applied wrong. Also, the new GA real time feature doesn't work on filtered profiles.
Basically, you do it a bit like this:
* Select account > Add website profile > Add profile for existing domain > Enter name > Edit.
* Filters applied to profile > Add filter > Add new filter > Enter name, ie. Exclude referrer spam.
* Select custom filter > Exclude > Referral.
* Now enter the domain name. You might be able to get away with a chunk of it, ie. spamstufftoyou if it's sufficiently unique, or you might need to use the whole URL, ie spamstufftoyou\.com
* Case sensitive "no". Then save changes.
