This can be tricky, because what may be innocent flirting to one person, is harassment to another. I've been on the other side of this awkward situation, and fortunately realized I was making him uncomfortable. But some people are oblivious, and you just have to tell them. Just tell them it's making you uncomfortable, and it will likely stop. People flirt to make you feel good, and if it isn't doing that, then they have no reason to continue.