I really think this person would have allowed you to have a section of the yard to yourself if you had asked politely. My experiences with PL'ers has been almost 100% positive when I approach them and ask if I can grab a few spawns here and there. In most cases they oblige, or just move. And in a couple cases they simply invited me to join the group and get PLed with them (thanks Sapentia
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That said, if you run into a rare group of PL'ers who disregard your polite requests, I'd say feel free to start peeling them off or petition.