Once an update/download has started, you need to finish the update/download in order to play the game or use the tool. It's like that in offline mode and online mode.
That's how it's worked for years and it explicitly says that under the info about offline mode about how to enable offline mode on Steam: https://support.steampowered.com/kb_article.php?ref=3160-AGCB-2555
If you don't want the texture pack tied to your account anymore, contact Steam support and ask them to remove it. Also, the pack is 3 GB, not 5+ GB. That's about the size of the vanilla Skyrim files if I'm remembering right. Meaning, it doesn't detect the files are valid for some reason or another from the sounds of it.