To avoid sending your subs to foreign network, i guess you could decrease your RXLEV ACCESS MIN to the minimal value.
I assume that could work ? As long as your BTS provides a sufficient RXLEV.
The MS will temporary and repeatedy try to join it home network if available (i.e. rxlev(home) > RXLEV ACCESS MIN)
The timer is about 30 minutes, if not mistaken (it’s an internal MS timer)
Regarding forbidding network… it can be applied only on new SIM’s, yeah ? You can’t update an SIM card through the Um interface, can you ?