How can I disable the adware pop-up for DMXZone that appears from time to time when I open Dreamweaver? (Other than removing the plug-ins, for which I paid, and never buying anything else from you, that is.)

Install DMX Updater, it is an extension that checks if there are new updates for products installed. There you can specify you want notifies or not for updates and new products.

If you don't want to install the updater then check this FAQ.

It keeps coming back again and again. Your suggestion does not help. In my opinion, DMXzone includes this annoying spam whenever an update of a DMXzone extension is done.
You should add a "dislike" button for your facebook account for this.

Next time that pop-up appears, click on "OK" (pretending you want to see the updates), then click on "Option" in the bottom left.
From that window, you'll be able to uncheck options and remove the pop-up from appearing.