There have been two instances recently where I saw a woman wearing a ring on her wedding ring finger, but it did not actually look like a wedding band.

The first one was a waitress, and it looked like a birthstone ring. The second was an older woman in a bar and it looked like a class ring. The second woman is interesting because I opened her before noticing the ring, and then let the conversation drop after... then once I was leaving she said she "hoped she didn't scare me away" and I think I caught her looking back at me as I walked away. (Honestly I mostly opened her for practice so not a big deal.)

So... do women wear rings on their wedding ring fingers as a way to reduce the number of guys who hit on them?

They certainly do. Assume they're single until proven otherwise as long as you don't invest too much time in them.