[plt-scheme] unexpected things in .ss files

We've asked our students not to use Comment or Test Boxes because we do 
things with their code outside DrScheme, but we're still getting 
submissions with weird things in them. One of them was something that 
looked like a number, but the cursor just moves over it. I got something 
like that with Insert Fraction, but the bad example was a decimal, not a 
  rational fraction. I finally duplicated the error by computing the 
number I wanted in the Interactions window, cutting the result, and 
pasting it into the Interactions window. It appears to be a snip. Is 
there any way to force pastes to just be text? Is there a simple way to 
process files to convert these type of snips to plain text? Thanks. --PR