I think you are referring to BSS-minrate and/or smart roam.
Check the WLAN settings and see if BSS minrate is set to 12 or 24Mbsp. Disable it.
However, client roaming to other AP is not something you can control from controller or from AP side. Roaming is a client decision.
Alternate way is, create dark zones (some area between each AP where there is no coverage) but this is not at all recommended, as I am not sure what is the customer use case for this kind of requirement.
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