A couple related idioms:
You can't put the toothpaste back in the tube.
This refers to an action that cannot be undone, such as returning toothpaste squeezed out of a tube back into the tube. Due to the way toothpaste tubes are produced, this cannot be done (or at least is very very difficult).
Letting the cat out of the bag.
This refers to something that was supposed to be kept secret, but was accidently revealed. This could be in reference to something that was supposed to be a surprise, but the recipient of the surprise found out about it. Often this is due to a verbal slip in which too much information was revealed.