I'm not too sure about the aim-assist part (cause let's be honest, without aim assist, K+M users would likely have a better advantage on aiming), but you CAN just turn off the crossplatform matchmaking in the settings menu; it's the first option below the "Game" toggles, so you can choose to play only with whichever platform you're using if you wish.
That being said, it will also make matchmaking harder since it will limit the people found to the platform you're using only, so it's a choice between either "shorter matchmaking but potential aim-assisted/mouse-using players" or "PC/Console-only gameplay but longer matchmaking." At the least, you have the option to choose which of the two you want.