Like the guys before have said, you can modify roaming aggressiveness on the client or modify data rates so that the wireless network disappears at a certain distance from the AP, but the key thing to remember is that the decision to roam is taken solely by the client device itself. The decision is based on a variety of factors that could include among other things signal strength, SNR and retry rate. The roaming triggers and deciding factors vary among client device vendors and is included in the client device drivers. Making sure that device drivers are up to date can often yield better roaming algorithms on the device, so I always start there.
