@OP Simplest way to "eliminate" the gallery tags is to create new gallery for new stuff and leave the old one to be or be expunged. And not replace contents of the gallery with unrelated stuff. Which is when some may throw the word "vandalism" around.
Otherwise, you - and anyone else - can vote down the tags just the way you vote them up: simply choose "vote down" option after clicking on the tag. When tag reaches low enough power weight, it goes into dotted border (low power, invisible from the overlay), and with further downvotes - vanishes entirely, as much as outside world could be concerned.
You can ask for help in tagging regards in the forums, "Vigilante" subsection; or just wait for residents to correct the tag field (which can take some time - like, years of it, in some cases).