you could change the password for the wireless or disallow her MAC address.
See the literature which came with your router for how to access its user setup screens. You'll need to note the default IP address of the router (usually in the range 192.168.0.1 though it varies by model). Connect the computer to the router with an ethernet cable. Open Internet Explorer and enter the router's IP address into browser's address bar as though it was a website.*
You should then reach the login page to the router's setup screens. The User name will probably be "admin" (without the quote marks) but the password can vary according to the brand of router **.
Once in to the router's setup screens, go to wireless security and review or renew the wireless security type and password. WPA PSK TKIP is the most common security type in use. Make a note of the SSID (the router's wireless call sign) or change it to one of your invention.