Valve stem locks include a plug element and a sliding latch disposed in a hole of the plug element. The latch includes a hole and is biased toward the locked position. A key inserted in a hole in the plug element moves the latch from the locked position until the hole of the latch is in alignment with the key. The key is then inserted into the hole of the latch until a notch on the key engages a lower portion of the latch. As a result, the latch is prevented from returning to its locked position. To allow the latch to return to its locked position, the key is removed by rotating the key to disengage the notch from the lower portion of the latch and sliding the key from the hole in the latch. Removal of the key frees the latch to return to its locked position.