warn (one) off (from someone or something)
To caution someone to leave or not to come near something. The first noun or pronoun can also come after "off." The government put up signs warning off any potential hikers from the area. I held my shotgun across my chest as I warned the ruffians off from my land.
warn someone off
to advise a person to stay away. We placed a guard outside the door to warn people off until the gas leak could be fixed. The guards warned off everyone in the vicinity.
warn off
v. To notify someone to go or stay away: The sheriff warned them off the private property.