How to prevent the client always connecting to the SSID with the weakest signal?
There is no way to prevent this situation if using a stand-alone AP.
However, if the AP is managed by NXC, then the answer is yes.
There is a function in the NXC to disconnect the client, which is connected to the SSID with a weak signal that is lower than the signal threshold you set on the NXC. After the client was disconnected from the AP, then client will send the probe request to search for other AP with SSIDs.
Go to the AP profile > Radio > press Show Advanced Settings > scroll down to the bottom of the page > find "enable signal threshold".
As the image shows, enable the checkbox "Enable Signal Threshold".
When the client has connected to the AP with a signal lower than -65dBm* (Disassociate Station Threshold), the AP will disconnect this client.
The client will then connect to another AP with a signal greater than or equal to the -60dBm (Station Signal Threshold).
*The threshold should be adjust with real environment.