The Scarlets struck back in the second half with two penalties from Daniel Newton, but tries from Christian Dacey, James Loxton, Rhys Downes and Lloyd Williams made sure for the table topping Blues.
Deacon added another conversion for good measure as the Scarlets found life increasingly difficult, despite home advantage.
Meanwhile the Ospreys proved far to strong for the Dragons at St Helens, winning 44-14.
Ryan Bayliss put the Ospreys on the board with an early try and they swept into a 17-0 interval lead thanks to a try from hooker Joe Beddow and two conversions and a penalty from last season's under 16 captain Daniel Biggar.
The Dragons hit back after the break with tries from Kyle Price and John Burgess with Danny Ellis adding two conversions.
However, the Ospreys proved too strong in the final quarter with Bayless adding his second try and Sam Williams and Scott Hancock making sure of the bonus point.
Biggar added two conversions and replacement Matthew Jarvis a third whilst Biggar kicked a penalty and a drop goal to finish with 17 points.