These do not necessarily cover all the angles but should provide a good foundation for your discussions. See if your questions fit one of the above categories before accusing someone of cheating or camping. Some common arguments include the following items. Map mods are NOT cheats. You simply must learn the map and its quirks.
Most modded maps have walls that can be shot through as a flaw in the programming.
This is not cheating since everyone can do it and you must play the map to learn which walls can and cannot be shot through. Modded maps with barrels you can climb into aren't technically cheats since anyone can do it, but it is considered cheap and most people do not like playing with people that do such things. Some people are very good at the game in general and some people know certain maps and strategies for those maps very well. Be very careful before accusing someone of cheating until you have considered the above alternatives. Yes, people cheat, but not as many as get accused of it.