I now know that it's only possible to sack a settle with a "Horde" because it's supposedly hard coded. But I've formulated a way to do so with a regular army if someone can simply tell me how to abandon a settlement? Here's my idea step by step...

1- Army attacks city...

2- Army defeats city defenses...

3- At this point you cannot "Sack", so you do the next best thing "Exterminate Population"...

4- You now occupy the settlement, finish the destruction by destroying all buildings in the settlement worth any money...

5- Abandon city* and move on to the next target!

Please someone help! I'm sure this would be useful for many people...