If the clients use CCX extensions you can influence them. Search in the Wireless LAN Controller Configuration Guide for "Using the GUI to Configure CCX Client Roaming Parameters" and "Using the CLI to Configure Aggressive Load Balancing".
Examples;
(WLC) >config load-balancing win ndow ?
Number of clients (0 to 20)
5 In the Scan Threshold field, enter the minimum RSSI that is allowed before the client should roam to a
better access point. When the RSSI drops below the specified value, the client must be able to roam to a
better access point within the specified transition time. This parameter also provides a power-save
method to minimize the time that the client spends in active or passive scanning. For example, the client
can scan slowly when the RSSI is above the threshold and scan more rapidly when below the threshold.
Range: -70 to -77 dBm
Default: -72 dBm