How do I remove specific items from my "Open With..." menu in High Sierra?
To prevent  a specific app from showing up under the "Open With..." for a particular file: 


Go to your Applications folder and right click on the app you want
Right click and select "Show Package Contents"
In the Contents folder, open the file Info.plist with your favorite text editor
Under CFBundleTypeExtensions (there will be multiple), find the XML properties with &lt;string&gt;file extension&lt;/string&gt;. 
Delete the XML property that matches the file type you no longer want to associate. (i.e. &lt;string&gt;gif&lt;/string&gt;) then save the file.
In Terminal, restart Finder (killall Finder)