やめって is "stop it" (a second person imperative = "(you) stop it") and やめろ is "let's stop it" (a first person command, let US stop it") A first person command is generally more polite in many languages ("Let's all be quiet" sounds less direct than "YOU be quiet") and it should be noted that these two expressions are both very colloquial. The more standard versions are やめて（ください）(or やめなさい) and やめろう

There is no such thing as "やめって", unless you're quoting the imperative of the verb "やむ" ("やめ" is a command, and "って" is separate quoting particle). I'll assume you meant "やめて" and "やめろ". "やめて" can be used as a command, but it has got many more uses as it is simply the "te" form of the verb "やめる". It's more polite than "やめろ", but less than "やめてください", ("やめなさい" is also polite, but much more categorical). "やめろ" is not polite at all (it even sounds harsher when you try to say it), and it is NOT a shortened "let's" form. There is NO such thing as "やめろう", correct "let's" form of the verb "やめる" is "やめよう", as goes with other ichidan verbs. The imperative form in context of godan verbs is created by changing the last vowel from "u" to "e" (くらう -> くらえ; のむ -> のめ), but in case of ichidan verbs, we replace the last vowel with "o" (たすける -> かすけろ; やめる -> やめろ).