In what way, would be a nice way to tell someone to *STOP* giving away all their junk to me?

Whenever you talk to her casually, just tell her you've been so busy downsizing and hauling stuff to the Good Will (or whatever.) Then immediately tell her that if she has anything to get rid of, to take it there instead because you really are enjoying the extra space and don't want to fill it with clutter again. And then repeat as necessary (I know, aren't these people a hoot!") I have them too, and have learned, like my sisters-in-law, how to just say "no thanks."