Looking at your banner, he's only about 4 months old? So he's probably still teething as well as being a baby exploring his surroundings. Both those factors mean that he's likely to outgrow his gardening activities, or at least find them less appealing, as he grows up a bit more.
Can you restrict his access to a limited area of the garden for the time being? I've got quite a large garden and with both my current two Cockers found it useful to buy some cheap plastic clematis netting and make a temporary fence to keep them in the top part of the garden on the patio, part of the lawn and just a small part of the planted areas when they were very young.
Another thought, you say you could stop it when he was younger but now he's too quick. Do you think he views it as a game where he tries to snatch something, knowing that you'll give chase? What happens if you try ignoring?
Hopefully you won't need to resort to a muzzle, but if you do, please have a look online about how to get him used to wearing one. Many dogs resent them and get quite panicky unless introduced to them gradually, or seem to accept wearing one but in reality become quite shut down.