macOS keeps nagging me about enabling Siri

Re-enable Siri and disable it again, but not from the control center. Use instead the Extensions Uninstall menu within CleanMyMac3, which can be downloaded as a free demo from the MacPaw website. It somehow does the job cleaner than Sierra.

Whatever you do do not install CleanMyMac, it is more likely to ruin your Mac than improve it.

This sounds like a broken preferences file.

Go to Finder, click the Go menubar item, click the option/ alt key and you should see Library pop up in the list, if it doesn't show try clicking Shift instead. When you see the Library click on it, a new Finder window will open, navigate to the Preferences folder and click to open. now scroll down to com.apple.Siri.plist, move it to your Desktop. Now restart your Mac, a new Siri plist will be created. Open System Preferences and click on Siri and make sure the Enable box is unchecked. If this improves the behaviour you can then trash the original plist.

If the doesn't work then you can try removing com.apple.systepreferences.plist following the same procedure. And then Restart.

While installing the clean installation of Mac OS Sierra an option comes , would you like to install Siri , if you unchecked the box Siri would not have installed .

And if you enter in system preferences and click on Siri and uncheck the box of enable Siri and again check it the pop you posted will appear .

You can again click on enable Siri .

Open a terminal window and type/paste this command:

defaults write com.apple.SetupAssistant DidSeeSiriSetup -bool True

Does it fix the problem?

This did not solve the problem in my case