I'm sick of receiving e-mails every time a thread I posted in gets a new reply.

from what i seeing, not possible turn off future, but link is there turning off already-subscribed. just going there every week or month for fix :)

Aside from that:

Settings -> General Settings -> Messaging & Notification -> Default Thread Subscription Mode:

A drop-down list of options should be next to that last one. Pick your preferred method.

Thanks, and shame on me for not searching harder.