Unfortunately, sometimes a fish is either mortally wounded or ill and just needlessly suffering.  Currently, there is no constitutional ammendment that requires us to put them on life support and/or supply a feeding tube.  So, if you truly believe that there is no hope for them and that they need to be put out of their misery here are two options based on the size of your fish and the materials that you have at hand.

• Clove Oil and/or Alka Seltzer:  bikeguy33 posted to the forum that the best way to euthanize fish is to add clove oil and Alka Seltzer to their tank.  Both chemicals act as an anethesia and literally put them to sleep.  8 Alka Seltzer tablets per gallon will provide enough concentration of anethesia to do the trick.  This is the only way to go for fish larger than about 3" or 4".
  
• Ice Bath:  For fish smaller then 3" or 4" an ice bath of 50% crushed ice and 50% water will cause them to go into shock and die almost immediately.

I have not personally used the clove oil or Alka Seltzer method, but it sounds like a more humane way to go about it.