Reproducable on current live demo, if you add two feeds to an RSS Aggregator, click on "import data" button, then browse the JCR Browser of the data module, you'll see two entries in the feeds and data folders. Now remove one of the feeds using the original dialog, go to JCR browser again, and you'll see that the feed descriptor has been removed, but not the data, see screenshot. Clicking on the "import data" button once more will not make a difference either.

When removing a feed, its data should also be deleted.