How can I remove a parking block from my garage?
In the installation instructions for water heaters that I could find, they all say that this "parking block" is required.


Installation, Operation, AND service Manual for Residential Storage Type Gas Water Heaters  


  THIS WATER 
  HEATER MUST BE LOCATED OR PROTECTED TO AVOID PHYSICAL 
  DAMAGE BY VEHICLES OR FLOODING.

Installation Instructions and Use &amp; Care Guide  


  NOTE: The water heater shall be located so it is not subject 
  to physical damage by moving vehicles or area flooding.
  



In fact, this is also mentioned in the IRC.


  International Residential Code 2012
  
  Chapter 13 General Mechanical System Requirements
  
  Section M1307 Appliance Installation
  
  M1307.3.1 Protection from impact. Appliances shall not be installed in a location subject to vehicle damage except where protected by approved barriers.


As well as the IPC


  International Plumbing Code 2012
  
  Chapter 3 General Regulations
  
  Section 305 Protection of Pipes and Plumbing System Components
  
  305.7 Protection of components of plumbing system. Components of a plumbing system installed along alleyways, driveways, parking garages or other locations exposed to damage shall be recessed into the wall or otherwise protected in an approved manner.


If it's not specifically mentioned in your local codes, most codes stipulate that the manufacturers installation instructions should be followed. So if the instructions for your heater have this; or similar text, you might be stuck with the curb.