[Skyrider]

It's a cheap tactic but not against the rules.

As womb mentions you could try bonk or

-Go pyro to airblast
-Go demo and spam stickies to push/destroy his
-Respawn in the other spawn
-Call for an uber
-Ask for help
-Tease the door and make him prematurely detonate them then push
-Quit the game and play solitaire

an L3 sentry in front of our spawn and propped the door open to fire inside

That however, is against the rules.

But the other can easily be avoided (as mentioned above) to spawn at the other location. Though very annoying if you get killed most of the time with the sentry shooting inside spawn.