If someone is actively preventing me from playing the game, I will kill them. I've never had this happen to me but it happened to a clan-mate while we were platooned. There were some fucktards in a Maus/loltraktor platoon and the loltraktor just griefed my clanmate's arty constantly. I think this is entirely justified. I'm here to win and/or have fun... can't do either if some asshole is constantly ramming me or whatnot.
Similarly if someone obstructs me (e.g., parks right behind my tank so that I can't back up), I will return the favor unless I receive an apology.
As for capping, I personally hate games where like 2/3 of the enemy is still alive and my team caps. Yes, it's a win. But it's incredibly unsatisfying b/c it was 100% based on luck. Both teams went for some kind of stupid bum-rush, and one team was retarded, allowing themselves to be held up by minor defenses and/or leaving entire flanks wide open. Doesn't take any skill or thought to pull this off, and it's boring. Thankfully it doesn't happen that often (except on Komarin, fuck that map).
And if there's just 1 or 2 enemies left, then yeah capping = more xp for all team members. Kill = more xp for the people doing the killing. If I'm on cap, it's pretty case-by-case for whether I'm willing to get off cap... but if it looks pretty guaranteed, I'm willing to let others try to kill everyone. So I'll hop off cap when I get near 100% and hop back on again to maximize the chance that the cap bar isn't 0 if my teammates fail.
But what I *HATE* about capping is how stupidly people go about it. It is NOT ok to run like 6 tanks onto cap and just sit there when you KNOW the enemy is coming back in force. Caps are incredibly vulnerable. Just sitting there means a few enemy tanks peeking out of cover/hull down/etc can reset you constantly while their arty eventually kills you. Meanwhile you accomplish basically 0. Worthless. Just b/c you make it to cap early doesn't mean you need to cap *RIGHT NOW*. It's often more valuable to proceed onward & kill off their arty/flank for beleaguered friendlies.
If you want to cap, you need to do your utmost to ensure that the cap process will complete successfully. Praying that the enemy cannot reset you fast enough/you get lucky bounces is fucking stupid. Piling behind dead tanks is fucking stupid unless the hiders are small & arty is gone. Of course there are exceptions to based on relative tank strengths/distances involved, but basically people need to use what little brains they have more.