If this is what you are doing...then just be direct. Be gracious, as in 'thank you for asking, but no thank you'. or 'thank you for asking, I am not interested in going out' You don't really need to give a reason. You really don't. If he presses for a reason, just repeat yourself 'thank you for asking, but no thank you'. 'Thank you for asking, but I'm not interested, and please don't ask again' (if he keeps asking)
When we're pressed for a reason, the gent might think of a counter argument to try and convince us. Just just say no, be direct, look him in the eye....use few words.
And, I'd have a private conversation with my boss to let him/her know you aren't interested in the gent and are feeling uncomfortable with their support of the gent. Again, be direct, use few words, but say what you mean, and mean what you say.
IF, after you have done the above (have been very direct in saying "no" to the gent, giving him two times to actually 'hear' you) and you've let your boss know.....the gent still persists in asking you out, your next response to him should be "I've told you "no" twice, don't ask me again. If you continue, I will need to take action through human resources." Again, be direct. Say what you mean, mean what you say. Don't mix words.