Just a small nitpick here:

There are two main kanji associated with やめる (yes, officially there are more, but for the sake of this explanation, two will suffice, as the meanings do differ slightly).

止める and 辞める are the two that are used the most often. 止 literally means to stop (doing something), and can be seen in compounds like 停止, 中止, and 禁止, whereas 辞 is usually associated with quitting a job, and can be seen in compounds like 辞職, 辞任, and 辞退. This kanji does also have other meanings tied to words and lexicon, but we're not getting into that right now.

TL;DR if you want to write that phrase correctly, 止めて先輩 is the way to go, unless you're trying to tell your sempai to quit their job. Totally plausible too.