Until he learns the "leave It" command and does it well I suggest if you don't want him in the mulch you either leave him on lead not allowing him near it or block the area off. If you continue to get after him and he doesn't listen he just learns that its Ok to ignore you.
Also if your in the Front yard and its not fenced he should be on lead anyway until he knows recall 100%. Just walk him out on-lead and when he starts to approach the area snap or tug the lead and say leave it and move to another spot. If he trots happily away with you without a fight I would reward him immediately after the leave it command with a pat on the head or better yet a tasty goodie......
