Good reason to fire employee?

I run my own skateboard company, it's a very small one and I'm basically in the beginning. I "sponsor" a few young skateboarding kids. A sponsored skaters responsibilities include spreading the word about the company and promoting it in any way possible. On Facebook today I discovered photos of one of my sponsored skaters going into a competitors store and partying. They also "liked" the company on Facebook. Do you think this is a good reason to let someone go? Thanks for your input.