gtk-text-buffer-remove-all-tags (buffer start end)

buffer -- a gtk-text-buffer object
start -- a gtk-text-iter iterator with the start bound of the range to be untagged
end -- a gtk-text-iter iterator with the end bound of the range to be untagged

Removes all tags in the range between the start and end iterators. Be careful with this function: it could remove tags added in code unrelated to the code you are currently writing. That is, using this function is probably a bad idea if you have two or more unrelated code sections that add tags.