You probably mean to report someone ingame? We are not affliated with nintendo or capcom, so this is not the place to officially report hackers/cheaters/abusers/general unnice guys.
There is an official way to report players, but it's gonna take some digging.
Search for "report monster hunter tri", there should be an article about reporting abusers/cheaters. I cannot say if it'll help any, but it's worth a try i think?
Alternatively, ignore said person by putting him on your blocklist.