How to clear the scrollback in the screen command?

This thread has the following suggestion:

In the window whose scrollback you want to delete, set the scrollback to zero, then
return it to its normal value (in your
case, 15000).

If you want, you can bind this to a
key:

bind / eval “scrollback 0” “scrollback 15000”

You can issue the scrollback 0 command from the session as well, after typing C-a :.
HTH.