I assume your land is posted. Dont know what your local or state laws say about poachers, but some states take a very strong position and allow farmers/ranchers to protect their animals with force, and prosecute heavily with no sympathy for offenders. I'd find those laws and paint them onto a sign right where they were standing. And get some real or fake cameras to add to the deterrant factor and/or post signs about video surveillance. I'd also invite the local game warden over for a cookout and beer!
Those are the people who give hunters an undeserved bad reputation.