Currently, you can hide a show from you calendar or progress page. Hiding from watched progress also hides it from your dashboard on deck. Hiding in those 2 places should work for most users.
This topic proposes a “dropped” status for a show. This would hide from the 3 sections mentioned above + add a “dropped” indicator in places such as the show summary page.AB commented
I agree with the need for this feature, but on the flip side, shows that I'm watching but have been on for a long time (e.g. The Daily Show) don't always show up in my progress page because I've watched a relatively small percentage of the show (I imagine this is why). Having the ability to mark which shows I'm currently watching would solve both problems.