these are all great suggestions.. heres my two cents on the subject:

kittenfemme; if you have the whole house to yourself the possibilities extend beyond the bedroom.. .the fridge was a good idea, do you have a bookcase? if so, thats not going to move in a hurry. some curtain rods might work.. but you don't want to pull them down , umm... the doors themselves? you could just tie your sub onto the door handle and shut the door (could be VERY effective if you tie the right bits
a build in wardrobe will have a bar for hanging clothes, this might be pretty strong.

while none of these are real options for immobilising someone, they do provide anchors. The only real way to immobilise someone (in my opinion) is to tie them to themselves !

ta
Eric