From Rachel's point of view, pre-rejecting is unlikely to work. He'll just deny that he's interested, and make her look like she's weirdly obsessive and imagining things.
What she can do, though, is discourage him. Avoid any situation that involves being one on one with him. Don't sit next to him at social events. Don't start conversations with him. If he follows her around, find someone else to talk with immediately. If he starts a conversation, keep it monosyllabic. Don't answer the phone when it's him. For someone in his position, anything that isn't borderline rude will be seen as encouragement.
However, a mutual friend can do a lot here, if they sit down with Andrew and tell him that 1) It's blindingly obvious to everyone that he's got the hots for Rachel, and he's embarrassing himself by the way he's trailing after her and 2) She's not interested, period.