How can I seal HVAC duct damper?
There are a few ways to accomplish this. On the cheap DIY side, you could cut a few pieces of tight fitting cardboard (stack them and tape them together) to create a plug. I would punch a couple small holes in it and make a rope handle to pull it out.

On the more convenient side, you could get a expanding plug. The link shows a 6" plug... perhaps that's the size of your damper?

On the industrious scale, you could locate the duct and install a couple more dampers in series. Having a few dampers should effectively restrict air flow. 

You could also potentially build a plug from from a bag and a can of expanding foam. Here's a video about a commercial product (instapak). Obviously you can do this yourself... but you might need a couple of practice tries to get the amounts right. Essentially, you would put the bag in the area that you want to seal and spray the foam in the bag and tie it off. You don't need or want air in the bag when you add the foam. Most cans of expanding foam come with a nozzle. You just want to insert the nozzle into the bag, spray it, and tie off the bag. Then you could set a board or something on top (or in front) of the damper so that the foam doesn't expand into a muffin shape.