I recommend you, using Wireless WDS Dynamic mesh
to forcing clients connecting to among of the nearest AP using this command
/interface wireless access-list
add authentication=no forwarding=no signal-range=-120..-73
add signal-range=-85..120
this command, allow only 73dbm or better signal to connect and kick only devices who fall to 86dbm and lower signal
I used this method on my project: http://forum.mikrotik.com/t/how-many-device-need-for-wifi-full-coverage/102213/1