Prevent plugin from updating

Plugins can not (if they follow standard rules) update themselves.
As for not asking to update, that is based on the Stable tag in the readme.txt. You could change it on yours to reflect the newer (or future)

I think the simplest option is just to adjust the version number in the plugin.

For example, after I customized an existing plugin, I then opened the code, looked for the version number in the header, and updated it:
1.1.2 –> 100.1.2

The “update” notice disappeared immediately, and I can still track which version the plugin is based on and apply updates manually.

Simon.