This is something I have been meaning to do for ages - since Adobe releases version 2.2 of Lightroom, I have had 2 sets of camera profiles in the Camera Calibration panel. I wanted to remove the older, beta profiles and just leave the Adobe ones and my own, created with the DNG Profile Editor.
It turns out that it couldn't be easier - you just need to delete some files and thats it! So close Lightroom then, depending on your operating system, open up the folder indicated below in Explorer (win) or Finder (mac) and delete all the files with "Beta" in the name. Start Lightroom and you should have a tidy Camera Calibration panel again!
Tip: use a search rather than ploughing through all the camera directories - it should be much quicker!
Mac: /Library/Application Support/Adobe/CameraRaw/CameraProfiles
XP: C:\Documents and Settings\All Users\Application Data\Adobe\CameraRaw\CameraProfiles