For the sniper view block, it would probably be best to be a pyro and use your flame to obscure his view in case he tries to move away. Or be a heavy and use your fat ass to block his view.
Also, a fun thing to do but it is difficult is to get a teleporter into the enemy flag room, get another engineer to build a teleporter from the enemy flag room back to your base and build lots of sentrys in their flag room. They will never be able to cap the flag. Loads of fun will ensue if alltalk is on.