In addition to what Christof and Edward suggested .. another option is this:

- add a report variable that counts the number of records printed
- if you don't already have a data group, define one that is appropriate for the page
- In the data group footer band, add a new field with an expression something like this: REPLICATE(REPLICATE('_', 50) + CHR(13), 99 - rnLines)

I just pulled numbers out of the air, but the "50" is how many underlines to print to make a full line; The 99 is how many lines fit on a page with no data. Also, make sure you mark this field as Stretch with Overflow.