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There is no direct way to do this. You could theoretically manipulate the sliders in such a way that they won't be upgraded but it's not easy.
 
That is disappointing. I have tried to use the slider and keep a restriction on the upgrade process, but it prevents aircraft and other vital equipmet from upgrading. Can a process be added as an enhancement to a future Patch?
 
You can 'prioritize' the ones you WANT to upgrade, and make sure that the slider is set to about the amount you need for those to upgrade.

Btw, in the new Doomsday expansion, a 'no upgrade' button is added :)
 
The tech support forum is not the place for gameplay questions or enhancment suggestions.