OK, We've all heard how the Romans destroyed carthage and then 'Plowed salt into the earth so nothing would grow there again."

OK. I've heard it. You've heard it. Is it true?

Because.... if it is.... How the heck did the Romans themselves colonize the place afterwards?

Short answer: "no"

http://www.romanarmytalk.com/rat/viewtopic.php?f=17&t=25007

I thought they just balanced it out with some pepper, oregano, and a little bit of cumin....

Without having spesific reference, I have heard that the romans had a seremony/ritual called "Evictio", which where basically a ritual looting of a city's temples, after which the gods (in the shape of the idols from the temples) where removed from the site.

With anticent technology, just leveling a city would take a LOT of work.