I need some advice on a potential wedding crasher. When I first joined TK there was a post by a person who sounds like she could almost be my potential wedding crasher, but I assure you it is a different girl because we are all heathens who do not attend church.

My ceremony in a few weeks will be in a public place (a park in the mountains). The actual venue we rented contains a small building, but we are planning on setting up chairs outside and will only have the ceremony inside in case of bad weather. We know there will be people hiking and biking past and maybe stopping to stare, I have come to terms with that.
The dilemma: I have an co-worker/casual friend who is not invited but keeps joking about how it is a public park so she is just going to show up at the right time and watch the ceremony. If she said this once, I would laugh it off. However, she has mentioned it on 3 different occasions (I see her about once a week at various social functions) and she is totally serious.
My current coping strategy is just to avoid telling her the ceremony time and exact location (somehow I have avoided divulging that info despite being asked), but she could figure it out if she tried hard enough (website, friends who ARE invited, etc.). She says "I know the reception is the expensive part, so I won't stay for that, but I just want to see you get married". Ack. I probably won't even notice if she is looking on from afar during the ceremony, but the whole thing is just so weird and I feel like if I saw her lurking around I would just invite her to come join us (we are having light snacks and drinks right after) because otherwise it seems not gracious. The thing is, she is a casual friend who would probably be invited if we were having a larger wedding (say, inviting 150 instead of just 30). So I don't want to be mean and serious and tell her to stop being a creep and make her feel badly and make things weird at work. And she is not dangerous just a bit clueless about social norms, so hiring security or something seems way over the top.
Sorry to ramble, wedding planning for my teeny wedding has been pretty fun and stress-free but this situation has me anxious! Anybody encounter something similar?