This OCMOD will disable all shipping methods except for Pickup, when the “Free Shipping” module is available during checkout process.

This extension has been tested with opencart 2.1.0.2. It may/may not work with other versions.

I will do some tests on 2.2.x.x, and a seperate version (if needed) will be uploaded soon.

Just upload using the extension installer, and you are ready to go. May/may not be compatible with other modifications.