There isn't really functionality in Crystal to do this, as I believe that processing for print and display are exactly the same.
You might try to cheat your way around it by creating a formula with a variable like stringvar html := 'print'
Then, when the report is passed to the web, change the value of html by passing a value of 'noprint'. You could then conditionally suppress your 'Back' field on the value of the html variable.
Good luck,
Naith