You're renting, so barbwire is probably out. But if you were to install 2" screws into your brace beam (likely a 2by4), so that they were only just sticking out that'd deter them. They'd get some nasty scratches and punctures and it'd probably be a loud night, but they wouldn't come back.
Even at a mm or 2 protruding those green screws will do nasty (but only surface) damage without costing much, pissing off your landlord or causing an escalation. It'll get rid of them real legit like, and will leave you knowing that you caused them pain.
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