Instead of やめて, you could say やめろ, which is the imperative, or an order. While やめて is not so hard. In anime, girls tend to use やめて more than やめろ. やめとけ is the abbrevatad for from　やめて　おけ. -て　おく is a grammatical form that adds a feeling of "it's best if ****" やめて　おけ is the imperative of this grammatical form, and thus it makes it pretty hard. shortening it to やめとけ makes it more informal than やめて　おけ, which is already really impolite, since it's an imperative xD. japanese people usually use the shortend form most of the time, also when they talk about themselves. ex: やめとく - "I'll stop" or "It's best if I stop" I hope this helps :)

やめて＝stop it! やめとけ＝you had better not do

やめて/ when you ask someone to stop doing something which bothers you. ex. 大きな声で話すのをやめて。 Stop talking so loudly. It depends how you say やめて, can be just joking between friends, but often means serious. Like やめて！ やめとけ(やめ'て'おけ） / when you suggest someone or give someone advice, means ' you'd better to stop.' You can just say 'やめとけ'(やめておけ）, also we often use it with verb such as 〜するのは'やめとけ' ( it's better to not ~(verb).) ex. それを使うのはやめとけ。 It's better to not use it. Hope it helps you :)