There is no way to lock a device to their closest APs but you can try a few things.
1- If these devices are stationery and will not move to a different area then you may create a new SSID and broadcast to just closest AP to that device (Though it is a possible way but not recommended as it is not scalable. You may have to create multiple SSIDs if you have many devices. If you have just one or two such devices then you may try this). 2- If you increase BSS min rate value for a specific WLAN, then it will help clients to connect to their nearby APs. Refer below article for more details. https://support.ruckuswireless.com/articles/000001657 3- Enabling smart-roam will also force clients to connect to their nearest APs. Refer below article for more information. https://support.ruckuswireless.com/articles/000002277
1- Was a pre or post site survey done, before or after deploying these APs? 2- What is the gapping/spacing between the APs? 3- What type of environment it is, home/office/warehouse/etc.? 4- Are these APs managed by a controller or running as standalone APs? 5- What is the firmware versions?
In most of the cases, a site survey is the key for optimized performance and coverage.