Why are you using the switch in L3 mode if you don't want it to route any traffic? 🙂
If you configure a L3 interface, you will always need to have a route to reach out to the internet and therefore we will make sure that you do not delete it as it would cause loss of service to the device.
Solution here, I think, would be to either move back to L2 if L3 is not required or create an alternative default route.
Hope this helps!
Please keep in mind that what I post here is my personal knowledge and opinion. Don't take anything I say for the Holy Grail, but try and see! Appreciate who helps and be respectful of every opinion and every solution offered. Share the love, especially the Meraki one!