Wifi won't turn off (at all)

Just switched to a T-Mob SIM from Straighttalk on Z3 compact. Wifi will not turn off (not a sleep mode thing - don't care about that) at all - comes right back on by itself! I know if I call T-Mob, they'd just act dumb, plus how could it be carrier-related? Xperia support had me do a softboot, but didn't help...
Anybody have any ideas, (or perhaps a simple solution) for a newbie??