Doh!
I didn't even consider to think it was a spell. I kept looking in the abilities files.
Presumably to reduce the attack boost from 1 to 0, I just have to change the <Value>1</Value> to <Value>0</Value> ??
Alternatively, I could remove the attack_stat_boost section from the spell?
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Attack_Boost</StrVal>
<Duration>1</Duration>
<Value>1</Value>
</GameModifier>