First of all, just to be clear: this isn't leaching, this is someone doing something nefarious. If they just wanted free bandwidth, they would never set up an evil twin network. Most of the replies on this thread are bad advice assuming it's a leech. The person responsible might be nearby, but probably not; if you track down the computer that's responsible, you're likely to find that its owner doesn't know what's going on and it's been taken over by an anonymous attacker over the Internet. Or you'll find a PwnPlug.
The first thing you need to do is notify the police that you're being targeted by hacking. This is important; if your computer/network is taken over and used for something illegal, which is likely to happen, this will protect you. Second: you need to notify your employer, as well as anyone whose confidential data you're in possession of. And third: you need to harden your computer security, and figure out why you might have been targeted.