Re: How to Say No
1. "Look, Rob (or whatever his name is), I'm happily married and that's not going to change. You need to stop asking me out because it's making it uncomfortable to train here."
2. If you think that your husband can do it without causing a scene, I'd ask him to talk to the guy outside the building after class sometime. If it were my wife having the problem, I would say "Look, Rob, you're making Justwanttotrain uncomfortable. Are you asking her out? Because she's married and that's not really okay. You say she has the wrong idea? Okay, but whatever you're doing is making her uncomfortable, so please stop." Or make a date and bring your husband.
3. It is absolutely your senpai's job to deal with this sort of thing. I wouldn't go to the dojo-cho first or even necessarily an instructor; I'd go to someone who's senpai to both you and Persistent Guy, preferably male (males talk to other males in a different language; it sounds like "ook-ook" to the untrained ear, but it's really very complicated), and someone who's always at practise, and explain the situation. Say, "look, I don't want to make a situation here, I just want to train". This sort of involving someone else should be appropriate in even the most conservative of dojo.
4. Keep going up the ladder if nothing happens, emphasizing that you don't want to make a scene, but just want to train.
5. If those things don't work, and if the dojo-cho tells you to take care of it, I'd quit the dojo and tell them that if Persistent Guy ever leaves, you'd love to come back.
6. To exhaust the possibilities here, I guess we should mention that of you giving Persistent Guy an injury that will prevent his coming to practise for a while. As Mark says, the guy is harassing you, and that has no place on the mat. However, violence also has no place on the mat. The idea that you are entrusting your safety to your training partners is sacred and necessary, and it would be absolutely wrong to break the sanctity of that trust. If your ethics permit, I suppose kneecapping him in an alley or in his apartment would be fine and would effectively solve your problem, but the risk of him trying to press charges is too great. I'd definitely say that it's not worth the risk.
I hope that you can get him to stop bothering you so you can just train. Good luck!