I have a 1999 boxster as well with the same problem. The short term fix is much easier and safer than removing the fuse. I called and spoke to Doug at Champion Porsche in Broward FLA and his suggestions worked like a charm so I was able to drive my Porsche to the dearlership.
Take a regular size business card and fold it 2-3 times, looking at the crest on your horn lift the back of the horn assembly ever so slightly, if the horn goes off you lifted it to much. Take the folded business card and wedge it behind the horn. Problem fixed for now. You can press on the horn to ensure that this patch is working and it should.
Cost was $236.00 parts and labor.