I'd say the cheapest way is to leave something disturbable (say a small pile of salt) far enough inside the door so that it won't be disturbed if you open the door just far enough to squeeze through. Your landlord will normally open the door wide and disturb the salt if he does come in. You can enter without disturbing the salt by squeezing in. Not sure if I've described that very well.

I had a landlady once who turned into a complete nutter. she used to enter the flat while we were out, threaten us, all sorts. We just skipped the flat and accepted that we'd lose our deposit. It wasn't a fair outcome by any means but some battles just aren't worth fighting.