Situation arose last night: Quasar with expanded hanger used boarding troopers on my Liberty. Now, the Quasar had a squadron value of 5 to my Liberty's 3 defence tokens, and my opponent asked if the spare two squadron points could be used to discard (after the first 3 squadron points had exhausted) two of my defence tokens.

I said not (there isn't a precedent for this kind of spillover) however am curious as to how the community answers this?

A token may only be spent once an attack... so from readied to exhausted.. therefore the 'spill over' points are lost

(the same as if you only had a squadron of 2 you would have to exhaust two tokens not discard one)

edit - if you look at the Sloane threads for the rules brakedown from @Drasnighta

I agree that you can only spend a single toke once with Boarding Troopers, but that's not because of the rule that during an attack a defense token can only be spent once, because Boarding Troopers isn't during an attack, is when you reveal a command.

Either way, it says that you can spend a number of its defense tokens up to your squadron value. So my understanding is: I have squadron value of 4, I can spend up to 4 different defense tokens but not a single one twice.

For example it would be like this:
-I use it on my opponent's MC30c, my squadron value is 4 so I say "I want to spend your Evade 1, your Evade 2, your Redirect 1 and your Redirect 2" and so all are spent.

And not like this:
-I use it on my opponent's MC30c, my squadron value is 4 so I say "I want to spend your Evade 1"
-My opponent spends it
-Now I say I want to spend the Evade 1 again
-My opponent discards it
-etc

Indeed. As instructed by the card, choose the given number of tokens, then spend them.