I guess in most places of the world that would be illegal, and your recourse would be to call the cops... or if it happens on the grounds of the theater, it's the theater management's problem.
Discussion
Do you think it constitutes a kind of theft?
I mean, not as such... In that specific example if you try to enter a space to which you have full right to enter and someone attempts to "block" you I would consider that something approaching either assault and/or imprisonment, i.e. kidnapping. Like, there must be some attempt at physical restraint involved in that scenario.