I had the same issue when importing a very old (2002) PhpBB forum and I added various regexes to the importer to remove (or maybe replace in some case) these BBCode tags. It worked quite well in the end.
I also had the [quotes] issue when I had to “move” them on their own line in order to be correctly displayed with markdown.
I found this in some personal logs, I believe it’s what I achieved for this particular issue:
text.gsub!(/(\[\/?quote.*?\])/mi) { |q| "\n#{q}\n" }
From what I wrote at this time, it worked for all quotes on 150000 messages except… 57 quotes for unknown reasons and such a small number of failed quotes didn’t really matter. I could even fix them manually.