In this case, we can only propose you to hide button "Upload New Plugin"
but before, You need to import all available plugins
Place this code into gridboxcore custom.css file
templates >> gridboxcore >> css >> custom.css
a.ba-btn-primary.upload-plugin {
display: none;
}
If your customer want to develop website without you and use PRO plugins, he need to buy subscription.
Want to notice, we didn't set any limits in domains for our products, you as a subscription owner can use our products without limitations. and of course your customers can use Gridbox (edit / create / delete etc.) but not import PRO features: Plugins and Themes.
Thank you for understanding