Not remotely possible.
The "final" decision to join which AP and/or which radio falls on the wireless client. If the wireless client wants to associate to the furthest AP then there's nothing you can do.
One way of minimizing this from happening is to ensure your wireless clients are running the latest drivers. On the AP-side, you could also try to disable the low-speed data rates (from 11 Mbps and below, make 18 Mbps as Mandatory and the rest are Supported).