This stalker will not leave my girlfriend alone?

This dude named David comes into domnios and gets pizzas all the time. He only comes in the store on the shifts that my girlfriend amanda works. He works at the garden center less than a block away from dominos. she always sends other employees to take his orders and give him his pizzas so she doesn't have to talk to him. He always asks to speak to her after he gets his pizzas, when she says shes busy he volunteers to wait untill she's no longer busy. He came up to her while she was outside smoking a few weeks ago and asked her a bunch of questions about herself and asked her out, but she told him no and that she had a boyfriend. He said to her, "Well, he can come too" and that really pissed me off. The other day one of my coworkers was walking to work and David stopped him as he ped the garden center. They were talkign about cars and my co worker said one day he wanted to own a mustang and David said, "One day, I want to date Amanda." He kept asking my coworkers if they knew her boyfriend and wanted to know if her boyfriend worked at dominos. My co workers told him none of his business. He has started comming in the store really late, about 30 mins before close. He does this because a lot of times the closing driver is out on a delivery so Amanda is in the store by herself. Hee's always lingering aroudn the store trying to talk to her even though she has told him she's not interested and that she's happy with her boyfriend. what can I do?