What you need is a rate-limiter on the speed of the door, like some kind of governor (http://en.wikipedia.org/wiki/Governor_%28device%29). Then the door can't be pushed open too hard.

Very firm damping on an automatic closing mechanism will do that for you.

Or non-linear damping, so the door is only hard to open if you shove it violently.

Or paint the door with percussion-sensitive explosive ;-)

I had a chemistry teacher who told a story about, when he was at school, getting hold of some of that for a practical joke on a teacher who liked to slam doors loudly. He put percussion-sensitive explosive around the doorframe before the teacher came in, with the intention that the door would slam, latch shut, and go BANG very loudly and startle the teacher. Instead, it went off before the door latched, so it sprang open again violently and knocked the teacher flying. Oops.

I think chemistry labs might have been better-stocked and less well guarded in his day...

Oh, there are plenty of stories of fun with NI₃ out there, even from my generation. Apparently it makes wonderful purple smoke.