Cayenne pepper on top of the soil between waterings. Peppermint oil can be sprayed on pots, bark etc. I have also seen people put drainage mesh over the tops of their pots and wire it down to prevent the digging or the theft of moss.
@Saffa365 Cayenne won’t work on birds, so if the Robins get too bad, then put screen door wiring over the top of your soil and be sure to secure it (easiest way is to place some small rocks on the corners).
You can get large rolls of screen door wire for pretty cheap at any of the big box stores.
I have the same problem, but with scrubs jays and nuthatches wanting to plant sunflower seeds and the occasional acorn too. There are some possible physical deterrents, which will detract from the appearance of tree unfortunately. It will be a trade off.
Suggested Barriers: 1) 1/4" Hardware cloth, 2) Scrunched up bird netting around the trunk, 3) Sheets of plastic needlepoint material, which is available from most craft stores, and 4) Sheets of bonsai mesh.
If you’re not squeamish and think of squirrels as rats with fluffy tails, then a live animal trap and a pellet gun works effectively. (Sorry PETA.)