We have someone in our group who believes that removing Blight tokens is an "Activated ability" meaning it can only trigger once from a declared attack, allowing only one Blight token to be removed.

I'm of the opinion that since the Bane rules say "one or more", I'm using the "or more" part of the Bane rules to remove multiple tokens at once, possibly eliminating all dice from the attack.

Which of us is right?

Your friends have been pretty consistently wrong so it should be a shock that they got this one wrong as well.

That spot in the RRG you mentioned allows you to spend as many of the tokens on a unit as you would like. Each token spent has the effect listed under its heading.

RRG, pg. 8:
"12.1 An opponent can spend one or more banes from a unit to
resolve a negative effect against that unit for each bane spent.
Each bane causes a unique effect, as follows:
• Blight: When a unit that has a blight token is
performing an attack, that token can be spent
before dice are rolled to force the attacker to
remove one die of his choice from the attack"

Nothing indicates that spending banes is counts as an "activated ability." And in any case, I see nothing about "activated abilities" in the rule book, nor any clause that says you only get one activated ability per trigger.

Now, someone could try to argue that the "or more" statement refers to different tokens because each token causes a "unique effect." But I don't think that use of "unique" is a game term; I think it's just plain English for saying that different tokens do different things.

You are right. Your friend is wrong.

Happy blighting.

are your friends warmachine players by any chance?