Best Weeds for Area-Denial?

[chess]

I realise this is probably an unusual question, perhaps even odd, but I'm quite serious. I live in western Pennsylvania, and I have a small lot of lightly wooded land that I want to make un-navigable for practical purposes, with as little effort on my part as possible. I am looking for ideas for fast-growing, fast-spreading durable, dense and generally antithetical-to-foot-traffic plant/weeds, so it will look and be far more trouble than it's worth to traipse through by the local riffraff until I am ready to do something with it. Nothing that will grow in the environment is off the table: thorns, allergy-inducing, the nastier the better.

So landowners, what's your nightmare weed?

[pmichelsen]

I have planted black berries in all of the areas I'm trying to keep people out of, not sure how well they'd do in your climate.

[knopfarrow]

I was going to recommend blackberries as well.

Probably the only climate they won't thrive in is dry/desert.  I'm in the Seattle area and although blackberries aren't indigenous, they are EVERYWHERE around here. One caution: Getting rid of them after they've been growing can be problematic. You gotta get roots and everything out or they will just keep coming back. The local solution for getting rid of blackberries is "goat rental"....

[Redoverfarm]

Multi floral rose or autumn olive.  Be prepared to keep them from invading your common area.  They will grown like crazy and once established nothing will like to penetrate them  Did I mention that I hate them.  Spray every two years to keep them at bay.

[hpinson]

Blackberries would do fine in western PA.  You may regret it...

Poison Ivy, but that is a delayed reaction. Bwa-ha-ha.