Second, change the name of your network (if it's linksys, change it to BotNet82 or something technical sounding) this should make just about anyone semi-tech-affluent think twice about connecting. (Don't switch to non-broadcast mode, as if they are actually cracking your passwords, they maybe able to do more malicious things if you are connecting to a fixed network name). Any network named linksys/dlink/etc is just asking for trouble, simply changing the name makes you less of a target.
Lastly, set up a Wifi address whitelist. It will be kinda technical to do, but you can basically choose which computers are allowed to be on your wifi by their mac address, which is a unique hardware ID given to your wifi card. If you set it up so only your computers can access it, then they will never be able to access it because they are not on the list (this is called a whitelist) but it makes adding new devices (such as a friends laptop) a pain. Alternatively, you can setup a blacklist, and choose to block their computers MAC addresses, however it is possible to get around this.
If you do all of these, things, you can easily keep them out of your network. Keeping their trash out of your yard is another problem entirely though . I wouldn't suggest retaliating though, since you may escalate the situation. (or get in legal trouble)
If you post what kind of router you have, I can try to find you a good guide for setting up a white/black list.
Also, disable wifi logins on the router, so that they cannot access your routers setup menu via wifi, you'll have to login from a wired connection; but if they are on your network, they can really screw with your router if they want to.
You can also see what mac addresses have been accessing your wifi, to confirm if or not they are accessing it.
Simply going through some basic measures. (though I've never had cause to go far beyond the basics for wifi security, as I live in a pretty rural area, with not many technical types nearby).
What other steps could be taken without completely shutting down wifi?
One possible solution would be to simply move the router to the other end of your house, (not sure if you can turn down signal power, but do that too if you can) so that they get weak/no signal from their house. (or put the router in a metal box, remove antenna's etc, try to make it not worth their trouble to connect).
If you really want to be a pest, you can make the network more trouble than it's worth, by turning it off periodically while they are connected, so they can never get anything done. Or you could simply start disabling it except when you need wireless (a week or two of that and they will probably get sick of trying anyway).
Periodically changing the WPA password at the same time would add another dose of spite
edit: Nice article on the subject of WPA2 and staying secure: http://netsecurity.about.com/od/secureyourwifinetwork/a... states that a 30+ character WPA key becomes impractical to crack, so simply using a long random password with a long unique SSID should be able to discourage them without toying around