First, set your wifi up with WPA2 as Chainzsaw said, if it's configured properly, it should be difficult for them to get in.
http://www.computerworld.com/s/article/9002706/Tutorial_How_to_set_up_WPA2_on_your_wireless_network_
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.