My best guess at this point is that this means "you definitely can't escape".
The English translation (which I've learned to take with a grain of salt) translates it as "Don't run off!"
In context, the addressee clearly is running away.
My understanding is that じゃん is a contraction of じゃない but has a meaning of its own. This comes mainly from Tae Kim's guide here:
Where he writes:
Though derived from 「じゃない」, 「じゃん」 is always used to confirm the positive.
Hopefully, you can see that 「じゃん」 is basically saying something along the lines of, “See, I’m right, aren’t I?”
The proposed “See, I’m right, aren’t I?” translation of じゃん doesn't seem to fit very well here as far as translating this into a natural sounding English translation, which gave me some doubt. But I'm thinking it can carry a sense of "definitely" or "absolutely" instead. Grammatically, it's a rhetorical question in Japanese, but it doesn't translate to a rhetorical question in English.
Actually, writing this question made me more confident about this (and I changed my mind half way through), but can anyone confirm?