I go for all tags, whether I killed the player or not. I don't care at all if a teammate grabs my tags. Better them than the enemy.
I pretty much feel the same, although if I see someone actually making the effort to grab their tags I will back off or even cover them while they do it. As long as the enemy doesn't get them, it's all good.
Blocking someone into a corner is a pretty bush league move though, TC. You're being even worse of a teammate to the rest of your team than that guy was grabbing your tag.
If I'm having a day I look for any excuse to trap people in corners. Makes for fun times. I definitely prefer to trap enemies in corners with my assault shield more but occasionally a team mate needs to be taught a lesson. Trapping enemies in corners with a shield is so much more rewarding.