I have not played yet so feel free to completely discount everything I say...
Discussions like this suggest to me that theres some good balance going on. I don't think its a matter of how do you counter this but what does forcing a player to teleport mean?
Obviously the respawn penalty is a bigger advantage but I think forcing a player to retreat gives you some advantage - in RTS terms map control.
From what I've been able to gather there is a cast time on teleport. From the moment you force a player to begin casting teleport your advantage begins. They are no longer able to meaningfully influence that part of the map. Hopefully you are able to tell when somebody is porting?
If so once you see that and you dont have your interrupt ready turn around and cap the flag or mince creeps or whatever it is you do in this game
Then all sorts of interesting tactics start to turn up where players are faking a port then pulling a surprise move or offense focuses on one player to force a port and create a numbers advantage. Things like that.
If its all about killing the other guy then things are a lot less dynamic. It probably also helps balance out Assassin vs Generals and other concerns.
If you really want to make sure you kill somebody then you have to think about holding onto your teleport interrupts and not using them the instant the cooldown is up.
Just my 2 cents 
Edit: Damn my thunder got stolen while i was being long winded 