Does your linksys router have multiple connection capability?
If it does (what I've seen is four) you would have to give her access, possibly an IP or account number for access through the router, a log on Identity and password unless you did a straight wireless "firewire" sort of connection which may still need a password for some areas and could be an open door letting critters in. You may need another account ID from your ISP as well if she's bypassing connection to the internet through your box, going direct.
You may also need to have some form of an "adduser" capability and you will want to look and see what kind multiple connections can be run under your system.
For disconnect I'd just power down and physically unplug the jack to the sender for your computer or if it's a stick sender, just unplug the stick if it won't lose any settings.
I'm guessing here a little, well may more than a little. But a physical disabling of the wireless would seem to be necessary for complete break. The router would always be on in case your girlfriends wanted to go surfing without you or look for someone else in the personals.

Doesn't matter what you say, it's wrong for a toaster to walk around the house and talk to you