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.