I had a similar thing happen towards the end of last season. A guy with his wife turned up just before faceoff (I'd been there since the doors opened) and demanded we move. He wasn't an ST holder, just claimed that we were in his "regular spot". We didn't move, but he pushed his way in next to us (where there really wasn't any space).
To avoid this kind of thing in future I'm putting a bit of money aside for next year's season ticket now. How does it work though, is it the case that you can either reserve a seat for yourself or a standing space, or can you get both? I imagine it's the former.