verlassen is used to for leaving a room, a building, a country, a person. This is different from desisting from doing something, which is what this DL sentence is (most likely) about.
Of course, you could translate it as "verlasse es!", if you were urging someone to leave a building or something. Not very likely, but possible, I guess.
das is basically "that" but is often used for "this" as well (and das hier / das da used when the distinction seems to be relevant).
"Leave this!" seems like an odd sentence to me; can you give me a context where it would make sense to you?
"Leave this here" or "Leave this thing alone" or "Leave this place immediately" sound OK to me but "Leave this" by itself sounds funny to me.