Years ago I had the same problem. Then I came upon the idea of creating bunches of fake lychees to try to fool the birds. I visited the local thrift stores and found a lot of decorative plastic fruit. Bunches of fake grapes and what looked like strawberries. I bought them and also found a bunch of wine bottle corks for like 50 cents for a big bag of them.
The fake grapes went onto my grapevine, the strawberries got hung among my unripe lychees, and I carved the little bottle corks into the correct shape and painted them bright red. They really did look like lychees when I finished.
Much to my surprise, the birds were fooled by this trick! Day after day I would find these fake fruit on the ground after the birds had tried them. I'd hang them back up again.
This game went on for weeks while my lychees and grapes were ripening. By the time the real fruit were ripe, the birds had lost interest and left the real fruit alone.
Might be worth a try.