Thieving neighbor stealing WiFi. How do I stop her?!?!?!?

Hi! I own a coffee shop and we offer free WiFi to our customers. We have a linksys and it is password protected. My business neighbor next door went out and bought a computer so that she could access our wireless system for free! The business neighbor on the opposite side of us came over and said that he... show more Hi! I own a coffee shop and we offer free WiFi to our customers. We have a linksys and it is password protected.

My business neighbor next door went out and bought a computer so that she could access our wireless system for free! The business neighbor on the opposite side of us came over and said that he could access our WiFi system and offered to pay half (yes! half!) of our monthly cost so that he could use it too. We told him that as long as he is a customer just like our customers that come in everyday and buy some coffee to use the internet then there is no problem with him using it. He comes in nearly every day for coffee and we have no issues with him.

This irritating woman however will buy something maybe 2 or 3 times a month and it usually one of the cheapest things she can find. When we change our password she immediately marches in and (no matter how many customers are in here) shouts, "Hey, did you change your password again?" She is nothing but a freeloader and I want to block her completely. Banning her fron the shop would be the next step but I'd rather keep business relation on the positive side if possible.

We have tried putting metal up around the router and wrapping it with metal window screen. The metal does absolutely nothing and the window screen works but it reduces the signal so much in here that customers have trouble accessing the system.

I have clicked Deny PC's in the router settings but that didn't work either.

What can I do????? She is driving us crazy!!!!!

Oh, also, if we don't give her the password, then she'll just ask a customer for it. She's a real gem :-{
Update: The router is a linksys but she is not using a MAC. Thanks!
Update 2: The router model number is WRT54GV8
Update 3: THANKS AARON!! I was able to single out just her computer and it's MAC address is now blocked :-) WooHoo! I'll let you know when I get positive confirmation from her that she can't get online.
Update 4: Hey J B! Thanks for your thorough answer and thoughtfulness :-) I wanted to follow through your instructions so that I have an alternative method should I ever need it but it tells me that the /all command is unknown. This is how I typed it: net view /all
Update 5: No news yet. She may have customers right now though.
Update 6: Nothing yet. This is odd. Do you know if there is any kind of error message that she may get tipping her off that we have blocked her? If so, she may just get mad and not come back in. If she doesn't come back in by tomorrow AM I'll change the password and then see what happens. I check it again and... show more Nothing yet. This is odd. Do you know if there is any kind of error message that she may get tipping her off that we have blocked her? If so, she may just get mad and not come back in. If she doesn't come back in by tomorrow AM I'll change the password and then see what happens. I check it again and she is on the disabled list (hee hee hee - CRAFT!)
Update 7: Oops, that last word was supposed to be CRAFTY!
Update 8: Well, it's 10:10am and nothing yet!!! This is beyond unusual. I changed the password to just to double check that and still nothing. She's there too because I've seen her walking in and out. I'll keep you posted. Happy Halloween!!!
Update 9: EUREKA!!!! 10:46am - Positive confirmation! NO-ACCESS! (big smiles) She came in, sweet as pie and paid her $3.20 tab from the other day AND bought a small coffee. Then, after a few flattery compliments regarding our flyer making abilities and how talented we are (transparent, huh?) she asked, "Did you... show more EUREKA!!!! 10:46am - Positive confirmation! NO-ACCESS! (big smiles) She came in, sweet as pie and paid her $3.20 tab from the other day AND bought a small coffee. Then, after a few flattery compliments regarding our flyer making abilities and how talented we are (transparent, huh?) she asked, "Did you change your password?" Even though I did, I said "No. It's still the same." Then she said that she keeps trying it but now she CAN'T EVEN FIND US AT ALL!!! She says she has been unable to access us for a day & a half now. BUT, she said that she has a friend who may be able to fix it for her. If that friend is knowledgable enough, can he change her MAC? I have 40 some slots that I can block her from if I have to. Thank you so much Aaron! J B, thank you too!! Months of irritation may be coming to an end ;-) Woo Hoo!
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