In my hack I have levels where the player must collect 8 red coins to exit, however when using screen exits, the red coins reappear when the area is reentered, allowing the player to cheat and exit without finding 8 different red coins.
You insert custom blocks in most cases as tiles, so they will always reappear, just like the standard coins. These only vanish, if you insert them as objects.
Well, there are now two ways to solve your problem:
1. Make a custom object with object tool from the patches section and let it use the level's inventory index (and your custom block, of course). I did not use object tool before, so I can't give you more hints about this way.
2. Change your custom block into a sprite and take a deeper look at this: