the 止めときなって means “quit ahead of time”
You could also render this as "Stop (whatever), I say!". A って at the end of sentences usually means something like "..., I say!", or "..., I tell you!", or "I told you, (whatever)!". Japanese is often a lot like (old-fashioned?) British English.
Also, the suffix ときな is used when making cute or otherwise soft demands of people (especially kids) and also pets.