Which FIDE rule forbids pressing the clock before taking captured pieces off the board? A long time ago I used to play OTB blitz chess at a chess club. We  had club blitz tournaments  [5 mins each with no increment because the clocks were old-school chess clocks]. I was new to blitz. I had a habit when taking off material, such as an exchange, of pressing my clock with my hands still on the board and being yet to take off the material. This ate into my opponent's time. One strong opponent who was an official chess arbiter once responded by pressing my own clock and saying "move on your own time". 

I agree that my habit was wrong. I tried to stop doing it. Where in the FIDE laws of chess would it cover this habit?
It sounds like you were breaking the rules on several fronts.

According to the FIDE Laws of Chess:


  6.2.3 A player must press his clock with the same hand with which he made his move. It is forbidden for a player to keep his finger on the
  clock or to ‘hover’ over it.


You would probably be punished under the illegal move rules:


  7.5.2 If the player has moved a pawn to the furthest distant rank, pressed the clock, but not replaced the pawn with a new piece, the
  move is illegal. The pawn shall be replaced by a queen of the same
  colour as the pawn.
  
  7.5.3 If the player presses the clock without making a move, it shall be considered and penalized as if an illegal move.
  
  7.5.4 If a player uses two hands to make a single move (for example in case of castling, capturing or promotion) and pressed the clock, it
  shall be considered and penalized as if an illegal move.
  
  7.5.5 After the action taken under Article 7.5.1, 7.5.2, 7.5.3 or 7.5.4 for the first completed illegal move by a player, the arbiter shall give two minutes extra time to his opponent; for the second
  completed illegal move by the same player the arbiter shall declare
  the game lost by this player. However, the game is drawn if the
  position is such that the opponent cannot checkmate the player’s king
  by any possible series of legal moves.


It sounds like you were either moving with one hand and pressing the clock - 6.2.3 or using two hands to make the move - 7.5.4