I figured that, and that is why I asked that specifically, just in case. Unless someone knows of an option to turn off auto-subscribe, here is a workaround.

Go into your Settings. There on the left under My Subscriptions you can edit your folders. Create a new folder just specifically for the threads you manually want to follow. When you then follow a thread, tell it to file it away in this one specific folder. Next time you go into your Subscriptions, (or, really, you do now with just the default single folder) there will be a drop down on the top right. Just select your newly created folder that only has your manually added subscriptions.

Anyway, one potential workaround.