Currently, there's no way to disable the Device Dashboard icon. If this is an option that you would like to see in the future, please leave your feedback for our development team here or directly on your Fire Tablet in Settings > Help > Feedback. We're constantly working to improve our customer experience and appreciate your input.

https://www.amazonforum.com/s/question/0D54P00007l1DcE/device-dashboard-icon-is-an-annoyance-pls-remove