Right now, I am using Podnapisi and Opensubtitles.org agents to auto retrieve subtitle for my movies. They work great except for few times when some subtitles are not found or fetched the wrong subtitle.
I could not find anyway to remove the wrong subtitles or search and select the correct subtitles via the Plex Web or other Plex clients.
At the moment, what I do to add the correct subtitle is to manually download them via my PC and place the file in my NAS.
I hope future version will simplified the process and provide better control over the subtitles.