There is absolutely zero enforceability. Sure, you could potentially stop some big corporates from transacting in bitcoin without paying this 15% tariff, but how are you going to stop individuals conducting bitcoin transaction on a p2p basis? That aspect of it simply makes no sense to me.
I suppose they could do it the same way the FBI sets people up on Localbitcoins. The police pose as traders, setting up P2P deals with high volume traders. Then they get those traders to engage in illegal deals like transacting with $10,000+ without KYC.
The Venezuelan government could set up trades, then see if traders pay the tax. If they don't, the police might come knocking on their door.