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The decision on which AP a client will connect with is based on client's wireless NIC's drivers. The only thing that you can do from the AP is decrease power and adjust (remove) lower data rates. But these two changes could affect your coverage area and create coverage holes. There are new standards (802.11k & 802.11v) that will allow the AP to provide information to the client to help them make better roaming decisions, but you have to ensure all the clients supports those amendments or it could cause connectivity issues. (More on 802.11k/v/r - https://support.apple.com/en-us/HT202628)
The 802.11 standard does not define when a client is to roam from one AP to another. For example Apple OS X will wait until the signal is -75 dBm or lower before it starts to search for "better" AP and then it won't change unless the "better" AP is 12 dB better signal. Each manufacturer and WLAN NIC will have different algorithms. Some of these settings can be tweaked on the client driver side, but it could cause adverse affects.