This type of attitude is pretty common... it runs along the lines of "To justify what I like, I have to hate/insult/denigrate what you like." As an example, "I like Star Wars, but Star Trek sucks." and by implication, so do you.
This is pretty difficult to counteract as trying to address it head on only solidifies their opinions. The thing to do (and I may not get this quite right) is to find out the what, where, why and how the other person likes Star Wars, without getting sidetracked into anything involving Star Trek. If they try to go negative, steer it back towards their positive feelings/thoughts about Star Wars. Do not at anytime talk about your opinions of either Star Trek or Star Wars. Only focus on them and their thoughts and feelings. If they persist in going negative, you'll probably get no where and will need to move on. But if you can get them to open up and get some actual, positive reasons for liking Star Wars, at some point you may be able to say, "Oh I see why you like it. Those are many of the very same reasons I like Star Trek, but I can appreciate that Star Wars is something you really seem to enjoy." Let them be happy about enjoying Star Wars on its own and realize that fact. Perhaps then they'd understand your appreciation of Star Trek.
Now this is not an easy thing to do - and the fact that I even wrote this out would confuse the heck out of some of my friends, but so be it...
Anyway.... I may add on to this later, but I need some sleep.