Just in case anyone else is wondering, these steps fix it for me.
If a quote, spoiler or any other "box" gets stuck
On mobile (I'm using Android 5 / 6 on a variety of devices all using Google Keyboard):
I refresh the page in my browser (chrome)
tap at the end of the message
hold down the delete key until all the text and the "box" are deleted
although this now seems to work for me every time, in the past I've needed to select a word by long tap or double tap and choose "select all" and then cut.
On desktop (using Chrome):
Refresh the page
When I hover over the "box" a little + appears in the top right
Ctrl-right click on that + gives a new menu. This doesn't work every time and sometimes you will get your browser's normal right click menu. Just try again.
Click "remove quote"
(I've just done this twice while writing this reply)