Hi,
Setting local-probe-threshold to 25 does not mean client with SNR less than 25 will get deauth from AP.
What it means is, AP will not respond to prob-request from the client if SNR is less than 25. This helps client to roam to the best AP.
For e.g.
If I am connected to an AP and if there is no other AP near by with SNR higher than 25, than I may still be there connected to same AP even if I start moving away and SNR goes below 25.
For your requirement (not allow far away clients to connect to SSID), what you can do is, reduce the cell size for the SSID.
Under SSID profile, cut down the lower data rates from basic and transmit rates list.
For example:
Cut down 1,2 and 5 from G basic and transmit rate and just start from 6,9,11....
This will not allow clients to use 1,2 and 5 Mbps data rates. In most cases if client is far away from AP, the data rate falls down. Thus if data rate goes below 6, client will get deauth from AP.
Based on your requirement select proper data rates here.