set your DHCP leases to 30 days and only have it give out 2 IP addresses in the scope (1 for your PC one for your PDA) that way if he connects he wont get an IP address so cant do feck all.

or some routers let you lock it down via MAC addresses, so only m,achines you decide can have access will have access.

more to the point tho...

IF YOU LEAVE AN UNSECURE WIRELESS CONNECTION ACTIVE YOU DESERVE TO HAVE YOUR NEIGHBOURS USE IT!