Thaxians were quite strong compared to Humans and Vulcans, easily capable of overwhelming them in direct physical combat. Thaxians were also immune to the standard anesthesia gas used to incapacitate intruders aboard Federation starships. They were generally tall and hairless, with elongated scalps.
Six months later, in 2265, the criminals commandeered the USS Enterprise and attempted to return home. Thaxian authorities preferred to shoot missiles at the Enterprise rather than let the rebels return. After the incident had been resolved, Kirk elected not to visit their homeworld. (TOS comic: "Planet of Rejects")