Get a hammer and whack the backplate behind the wheel. The brakes usually then come right off. The shoes have most likely got wet and have stuck to the drum, so even though the mechanism retracts when you release the handbrake the shoe or shoes stay stuck. This tends to happen more on the N/S than the O/S. If they don't come off it's possible that the brake shoe lining has broken away from the shoe. The usual sign of this is the wheel will rotate about half a turn and lock up, and then will rotate the other way half a turn and lock up again. Peugeot and Citroen were terrible for this. I've attended loads of them when I was an AA Patrol.

I'd try a blow lamp. Get the drum as hot as you can - this will make it expand, and keep hammering. Do let us know how you get on!

Heat is an excellent way of helping to release seized components especially when 2 or more are made of different materials and expand at different rates. Problem I see here is the simple fact that a brake drum is quite a big area thus its going to be a struggle to heat it up evenly and sufficiently to work. Might be worth squirting some petrol (or other flammable liquid) inside the drum to assist the heating up process. Might not do the linings or brake cylinder seals any good (a blowlamp on its own might cause problems) but its got to be worth a try.