You have to be blunt. Unless your co-worker has asked you out and you have just stared off into space, then you have had some response. Sometimes if you are attempting to save face you may have attempted to flex your no muscles but with a weak response. I mean I get it, you want to have a bit of decorum. This is the I don’t want to be bothered by you, but I want to continue working like this didn’t happen response. This works at least 80% of the time. Its only when that few 10% that you have to step out of the cute no to a plain old fashioned no.
I know you don’t want to rock the boat but truth be told, jobs can turn into great relationships but that’s not what you are there for. You are there to work and make your coin. Now for all my I just love little Suzy that sits in the cubicle next to me, please understand that not all working relationships lead into that. Your company size too does matter. If you are in a small office, you may have to pull casanova to the side and let them know that you would prefer to keep things strictly professional. If that doesn’t work than the next time the advances begin just say no and walk away. Your next course of action will be to make sure your interactions are strictly about the job. You can’t be at the job saying no but then when you think its no big deal make any gestures that this co-worker has a chance. If that doesn’t work and your co-worker is pushing things too far, find out what the harassment policy is for where you work. If you work at a large company you may be able to get away with avoiding the co-worker altogether.
If you don’t want something to occur between you and the co-worker you have to know that there is a course of action in place. No usually stops it. No followed up with action that show that No is the final answer usually seals the deal. Now for the 5% that find that no matter what they do they just can’t get the No to be enough, follow up with your supervisor or human relations department. They should be able to offer some sort of support that can get the co-worker to stop. From there, they will direct you into where else you can take it and the steps to get there should you have to exercise your right to be able to work without harassment.