Stop shifting to the Granny gear. Done.

Really, unless your crankset is worn to the point of needing replacing anyhow, going from 3x to 2x is a considerable cost/effort with very little gain.
Apart from being able to say "I run 2x".

To make it work like it'd been built as 2x you need a new crankset, maybe a new BB, possibly a wider range cassette, and a new shifter.