You have to use set %giveaway instead of var %giveaway to make !draw work.
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.