While I don't have the system (and not many others, judging by a month without an answer), I'm not sure which rules version this question relates to, and no excerpt/explanation of the spells and magic mechanics are given, I've found a house rule on page 8 of Jörg Jahnke's Rolemaster House-Rules relating to "Cancel Channeling spell of the Cleric" that sounds similar to your query:
The 'Cancel Channelling' spell of the Cleric has a duration of 1
min/lvl instead of, as listed in Spell Law, a duration of 'C'.
(Background: The same spell also exist, also at 2nd level, on the
Paladin Base Spell Lists and also on the Priest lists in Channelling
Companion. In both cases it has a duration of 1 min/lvl. There is no
obvious reason why it shouldn't have the same duration for the Cleric,
especially as the spell seems quite useless if concentration is
There might be an errata that could help, somewhere, or you could house rule to make the spell more useful like the above. Regarding the requirement of knowing you're the target of a spell in order to cast "cancel", it sounds like it fits the reactive-sounding nature of the spell; are there alternative spells that can be cast pre-emptively, to reduce success chance for a given type of spell? E.g. "I think I might be going into a room with water nymphs, I'll cast 'protection from water spells'"?
The main consensus I've noticed from my looking into this is that Rolemaster is the kind of old, complicated system where little niggles/minor broken things haven't been ironed out completely, even in later versions; some common sense/house ruling seems to be required to keep the system running smoothly.
Sorry if this isn't the kind of answer you were hoping for, but I don't know the system beyond freely available internet articles.