I completely understand you mate. I've lost count of how many times some daffodil had walked into a shot and ruined it, especially when is was meticulously set up, or if the moment could only have been captured at that window of opportunity. Some people just stand there, completely oblivious to to fact that you were there first and is in the middle of composing a shot.
This is one of the reasons I hate shooting in a public area with a high concentration of people. Parks make for great shots, but when some daffodil-head walks into the shot its annoying as hell.