one of a few things
1. ha **** you I have a date (she's trying to spite you for some reason, would make sense if she's a spurned lover)
2. i have a date, aren't you jealous? (could either mean that she wants your attention because she's into you and thinks you're not into her or she wants you to chase for validation )
3. I don't need you, I have other men (she's just letting you know she's uninterested and unavailable )
4. she's just saying that because she didn't even consider that you'd be an interested suitor and is just telling the truth, which would essentially mean you were friendzoned