Navigate to the overview set the card is in, click on Collection Summary then Add / Update. Find the card you wish to un-tag. From the down arrow beside the card# select Edit Details ... . Below the regular info box (Qty., Price, Serial Number etc) you should see a smaller box Tags. In that box should be able to see your tag(s) for that card, click on the X beside the tag(s) you wish to remove. The page should refresh itself and the tag will be removed. No need to save, or submit anything.
You can then verify the removal by going back to the set overview, find your card from the checklist and view your card to see that the tag is no longer attached.
"They've done studies, you know. Sixty percent of the time it works every time." - Brian Fantana from the movie Anchorman