This is NOT impossible in regular half-life. All you'd need to do is teleport the player into a small back box that's been turned into one func_button, so when you press the use key it triggers another camera entity and the sequence stops. That easy.
Actually it is possible...with some tricky and risky mapping
1) Make a black box with enough space for the player to fit in.
2) Teleport (lets name this teleport1) the player into the black box when your cinematic stuff starts.
3) Place another func_teleport (lets name this teleport2) above the player in that small black box. So, when the player jumps(default - spacebar) he will enter the trigger_teleport.
4)Now we need to trigger_off the cinematic. Cover the teleport2 with a trigger_once. (it should work, if not I've got another solution)
5)Make that trigger_once, trigger_off your cinematic or other entities you were using for your cinematic. (including teleport1). Also like srry said, if you have guys walking around, they'll have to be "killed".
6)Place an info_teleport_destination at the exact same location where he got teleported into the black box (with teleport1)
7)FIN
Sorry if it sounded complicated, its really easy actually. Maybe I can make an example map if you don't understand.