Ther
yep taint does not exist or spread the only way for it to appear is through instableitly
There is taint that can randomly spawn in a world, but such can be disabled or the rarity can be edited.
I had 'found' a area of taint near my base, or rather didn't find it, it found me. I settled in an area that looked nice, but was JUST close enough to a naturally spawned taint area that the chunks were loaded but I didn't see it in game, as it was up over a extream hills.
I had spread on... needlessly to say it crepped up on me. I never went over the hills, I went through them... mining along, after a bit I found my mineshafts fully changed to taint and the hills eaten up. I had NO means to fight it and by then it was HUGE.
So yes even for me in SSP, spread is OFF for sure... that stuff spreads like weeds.