You have to use set %giveaway instead of var %giveaway to make !draw work.
Code:
on *:TEXT:!giveaway start:#:{
  set -e %giveaway on

on *:TEXT:!giveaway end:#:{
  set -e %giveaway off

...

write $+(-ds,1) giveaway.txt will clear the line that contains 1 in that line.
To clear giveaway.txt, use write -c giveaway.txt.