Disclosed herein is a security apparatus comprising a blocking device having a formable bag and a marker layer including a transferable marker composition that can be transferred onto any object penetrating the marker layer and any other object or person coming into contact with the marker composition in the event of an attempt to remove the blocking device by puncturing the bag. The blocking device may block an access point such as a manhole to block unauthorized access. The formable bag may be made to conform to the shape of an access passage by inflation or deflation using valve.