currently Hudson doesn't remember between restarts that a slave is marked as "offline". It'll be useful to have this information saved.
Code changed in jenkins
User: Nathan Parry
Persist a user-requested offline status for a slave across
restarts. Previously slaves would come back online if the server
was restarted while they were marked 'temporarily offline'.
I believe this also addresses JENKINS-7934 and JENKINS-7805
which look like duplicates.
The fix is included in 1.397
I didn't get around to upgrading until 1.399, but I've confirmed that the fix works in 1.399 so I'm closing this issue.