I believe sperm plugs are excreations of sperm that dry, causing what your chamleons "seems" to have. It should eventualy go away, or get pulled out...But hopefully some other members on the forums will enlighten you about that matter.
And here's a photo of dried smegma. What you may find in his enclosure attached to a vine or something. In the photo and description Kinyonga directed you to, it appears to be actually plugging the vent and causing serious problems.