How can I disable media playback in Finder document icons?
The Feature is controlled by QuickLook and therefore you need to either disable the corresponing movie-generator or replace it with a better one.


On 10.11 and higher: Disable System Integrity Protection (SIP)
Go To /System/Library/QuickLook
Compress Movie.qlgenerator  


Move the created zip from your desktop to the folder
Confirm with password
Delete the Movie.qlgenerator
Confirm with password

Download qlMoviePreview
(Here is a Download Link for the Lazy)
Extract and Copy the qlMoviePreview.qlgenerator to /System/Library/QuickLook


Confirm with password

Open Terminal and type qlmanage -r; qlmanage -r cache
Maybe a reboot helps for Quicklook to change completely


Now the Finder Movie Thumbnails show an image but have no longer a playback button in it.