I'm assuming you're using Windows 10 and the ImageGlass 6.0.12.29. If either of those assumptions are incorrect, let me know.
First, you want to establish a different program as the "default image viewer", if you haven't already done so. Ignore the fact that ImageGlass may still be listed as the default, and select something else, such as Photos.
Second, to stop ImageGlass from appearing in the available choices via "Open With \ Choose Another App", you need to use the Registry Editor (regedit). In Regedit, under "Computer\HKEY_CLASSES_ROOT", you will find in the tree a bunch of entries like "ImageGlass.AssocFile.3FR", "ImageGlass.AssocFile.ARI", etc. One-by-one, you need to select an entry, right-click, select 'Delete', then click 'Yes' to the "Confirm Key Delete" dialog box. Alas you cannot do it in bulk via Regedit; I suggest deleting only for those extensions which are important to you.
I have logged this as a bug in the ImageGlass repository, and try to create a .REG file to do this cleanup work "soon".