It seems like you're talking about straight up abductions and slavery, which is illegal regardless of status of prostitution.

According to http://www.humantrafficking.org/countries/thailand:

"Trafficking victims in Thailand are found employed in maritime fishing, seafood processing, low-end garment production, and domestic work."

By your logic Thailand should ban maritime fishing, seafood processing and domestic works.

Prostitution in Thailand is technically illegal and practically murky [1]. It does not support your assertion.

[1] http://www.thailawforum.com/sex-crimes-in-thailand-part2.htm...

Prostitution is NOT legal in Thailand, it's just that the law against it is not enforced. If you want an example of a country where prostitution is actually legal you have to look at the Netherlands, where there is very little if any human trafficking. Nevada is another example. (And, by the way, I'm not a libertarian.)

The Netherlands does have a significant human-trafficking problem, though they have been somewhat successful in recent efforts to crack down on it: http://www.state.gov/j/tip/rls/tiprpt/countries/2014/226786....