Here you will be able to find a script which removes the Proxmox's subscription warning popup.
Me publishing this goes a bit against my own principles. I am a strong supporter of Open Source Software, which Proxmox is, and I would love to support them with a monthly subscription, but their current plans are simply way too pricey for a broke-ass student. And while I understand that the popup is way for sort of nudging the users to buy a subscription, I find them too annoying not to remove them.
The problem with most scripts published online is that they do a poor job and are in most cases outdated. One of the top hits on Google (currently, for me) is a command that disables the popup, but does not execute the function that should be called when you click the "OK" button, which makes all actions that would open the popup unusable.
I am a constant PVE user, so I'll most likely keep this post up-to-date - but if you notice that the script doesn't work, tag me on Twitter.