arangod> db._create("users", { keyOptions: { allowUserKeys: false } });
[ArangoCollection 1533105490832, "users" (type document, status loaded)]

arangod> db.users.save({ name: "user 1" });
{ "_id" : "users/1533106867088", "_rev" : "1533106867088", "_key" : "1533106867088" }

arangod> db.users.save({ name: "user 2", _key: "myuser" });
JavaScript exception in file '(arango)' at 1,10: [ArangoError 1222: cannot save document: unexpected document key]
!db.users.save({ name: "user 2", _key: "myuser" });
!         ^
stacktrace: Error: cannot save document: unexpected document key
    at (arango):1:10