Try clicking the Stop Sign! It won't actually stop the project; instead, it will just be detected and let you know when you've clicked the stop sign! I found this trick while scouring old forums.
On a forum from something like 2015, I saw that someone (No, I don't remember who, and they're inactive now anyway) had posted a way to create a 'when stop sign clicked' script. I came up with some more script, and there you have it; an unstoppable stop sign! Added clicker 9/29/2023 Why this must be real- Most detectors (Love and Fave, etc.) detect when the mouse pointer is touching whatever it detects-however, this is obviously not the case, since this detector doesn't stop when you click the stop sign. No, this project is 100% legit; there are absolutely no workarounds. HOW IT WORKS-have you ever noticed that the timer in Scratch keeps going when you click the stop sign? Well, when you click the Green Flag, this projects runs a script that resets the timer faster than the project itself can detect it-however, when the stop sign is clicked, a When Timer<0.00001 script activates, since this script isn't dependent on the Green Flag being clicked. When this happens, the script simply starts detecting again, over, and over, whenever you click the stop sign. The clicker works the same way, it just changes a variable if you're in the second backdrop.