Do you even need wifi? Just turn if off if you don't. As thor said restrict wifi access to approved mac addresses. Change the password, make it hard. Some also allow you to turn the power down. Only use as much power as you need for your devices. And of course turn off SSID.
Some other ideas (once you decide to play with them):
Change the name of your Wifi to FBI Surveillance Van
If it’s a managed router, change all your computers IP’s to static, and make them an illegal sequence, that way no one can see them from the outside of your lan.
Don’t use your wifi for a while and autoforward all IP requests to some nasty Russian porn site. That’ll make sure anyone using it won’t have a functional computer for long.
I am not being serious, although all of them are possible.