If they're just giving you a silly grin, just ask them what their grinning about. They'll be embarrased and get back to training. If the flirtation is more overt than that, you can simply say thay you came to learn aikido and not be hit on. If the flirtation turns into something more such as groping, tell them to never do it again and then tell your sensei. Groping is not acceptable behavior.
You make a good point though that flirtatious behavior comes from both sexes. It is good practice to keep hormones off of the mat.