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I thought I could just filter traffic from domain www.othersite.com, but that is not what that feature is for. It would be if I had GA on www.othersite.com and wanted to not count traffic on www.othersite.com.
However, you should know that this is unreliable. The other site could be referring visits that are not able to be identified as coming from that other site. The most common reason for this would be the other site's link being set up to open your site in a new browser window. Visitors using IE won't be identified as coming from that site.