First of all I'd like to thank everyone for their examples on how to deal with PCs going outside of the "boundaries" of designed questing. I have some ammo for the next time I feel like putting them in a quest line when they're feeling rambunctious!
The idea of asking them their plans for the next session is huge, never thought to do that before and will definitely help with preparation as opposed to my usual on-the-spot drops. I have used old characters from other campaigns, no low level party wants to be on the bad side of the high level characters you've developed in the past.
In an open campaign with the premise being solely to spread chaos the PCs were forced into service by NPCs they could not compete with. When they tried to overthrow their shackles too early in the campaign they were tortured with the severity related to the level of opposition. One was paralyzed from the waist down, another partially blinded, another lost a hand, spellcaster a tongue, etc. They then had to continue the campaign with these disabilities and roleplaying through and questing to make up for their defects was quite fun! "You got new legs Lieutenant Dan!"