Can I face jail time if I do this?

Wow, that's the pot calling the kettle black for sure. you do realize you are angry because he bred your mare WITHOUT PERMISSION... and now you want to geld his stallion WITHOUT PERMISSION. That is the most hypocritical thing ever... PLUS the vet will not preform a procedure on a horse that isn't yours. AND your little friend could sue you and you could land jail time. I understand you are frustrated, but that is absolutely ridiculous, not to mention completely childish.