The most efficient way to remove a spot colour is to open the file in Illustrator and select the pantone/spot colour. Then simply convert the colour (by clicking the small symbol underneath the colour picker).
If you do not have access to the original file, you could try simply preflighting the file, using the preset PDF/X-1a:2001. Click here to find how.
For more information about removing spot colours from your PDF, please see the Adobe Forums.