You can't guarantee it. When you delete it, Lemmy should send an ActivityPub delete activity about this comment/post so the deletion should propagate over time.

I have not checked that Lemmy implemented properly that part of ActivityPub (I know it is at least kind of working). And also, this does not prevent another instance from not acting on this. And if these instance don't care about GDPR or any similar regulations, then you are basically relying on their good will.