Beginners often assume that if you handle them gently and carefully, it's because you're weak. Being female, small, old, or young makes this even worse. It's hard to confront them directly on this topic. One of my teachers responds to it by calling up the senior women and throwing them around fast and hard. I find that once a class of beginners has seen me attack all-out and take ukemi, they are much more willing to assume I know what I'm doing. With particularly troublesome groups of beginners, he will also call up a senior woman and a really big man (not a beginner--someone with good ukemi) and ask the woman to be nage.
I like this approach; it's not confrontational and it speaks very clearly. Perhaps, if your dojo culture allows, you could suggest it to your teachers.