The plug comprises a body (1) that can be permanently coupled to the mouth or neck of a receptacle containing an air freshener or insecticide liquid. Evaporation is effected by means of a wick (11) axially coupled to a concentric passage of said plug. An inner part of said axial passage of the plug, in which the wick (11) is located, has two notches (7) in the direction of the generatrix forming sectors (9) fitted with semi-pyramidal ribs (12) which define means enabling displacement in the direction of the insertion of the wick (11) and preventing displacement in the direction of removal or release of the wick (11) relative to said plug, thus preventing the receptacle on which said plug is applied to be refilled with a product different from the original product.