Q: stopping computer from changing wifi

I've got a network of mostly new (2014 & newer) MacBook Pro's. The network is new, Meraki through and through, with theoretically plenty of strength for all areas and machines. The AP's are MR42. There are 4 SSID's from the same radios. The radio spectrum is wide open. Average utilization is never more than 10% even on heavy days.

The computers generally have a signal strength of greater than 37dB on AP 1, which is where I'm focusing this question because the Accounting people are on this AP, and that is who the primary problem is with.

The issue is that the accounting machines are constantly switching between the wanted WPA network (Computers) and an open (Guest) network. When the accounting machines switch networks, the IP range changes, and the accounting software crashes.

I need to get the computers to stop hunting for the other SSID's - as the SSIDs are coming from the same radio, it isn't hunting for a better signal. It's just hunting for some other reason. The message is just "802.11 disassociation client has left AP" and the RSSI number when it re-joins the same or a different network is always above 38.

Any idea how to keep the machines on a single SSID?

(as a side note, I can't turn on the require password to switch networks option because that would impact the laptop's ability to be mobile.)