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.mkv files can include the subtitles via an extra track, .avi files either use burnt in subtitles or have an extra file. Or can you deselect the subtitle in VLC or Movist while the .avi plays, but don't have an extra subtitle file in the same folder as the .avi?
 
if its hardcoded into the avi theres really no way to get rid of it. usually there is a sep file used to show subtitles, simply turning the feature off gets rid of it.
 
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