Remove disk image from the disk list

hdiutil will show information about all mounted DMGs:

hwd@hwds-iMac:~$ hdiutil info  
framework       : 415
driver          : 10.11v415
================================================
image-path      : /Users/hwd/Downloads/mountedImageOne.dmg
image-alias     : /Users/hwd/Downloads/mountedImageOne.dmg
shadow-path     : <none>
icon-path       : /System/Library/PrivateFrameworks/DiskImages.framework    /Resources/CDiskImage.icns
image-type      : UDIF, nur Lesen, komprimiert (bzip2)
system-image    : false
blockcount      : 22528
blocksize       : 512
writeable       : false
autodiskmount   : TRUE
removable       : TRUE
image-encrypted : false
mounting user   : hwd
mounting mode   : <unknown>
process ID      : 5376
/dev/disk2      Apple_partition_scheme  
/dev/disk2s1    Apple_partition_map 
/dev/disk2s2    Apple_HFS   /Volumes/mountedImageOne
================================================
image-path      : /Users/hwd/Downloads/mountedImageTwo.dmg
image-alias     : /Users/hwd/Downloads/mountedImageTwo.dmg
shadow-path     : <none>
icon-path       : /System/Library/PrivateFrameworks/DiskImages.framework/Resources/CDiskImage.icns
image-type      : UDIF, nur Lesen, komprimiert (zlib)
system-image    : false
blockcount      : 20480
blocksize       : 512
writeable       : false
autodiskmount   : TRUE
removable       : TRUE
image-encrypted : false
mounting user   : hwd
mounting mode   : <unknown>
process ID      : 5416
/dev/disk3      GUID_partition_scheme   
/dev/disk3s1    48465300-0000-AAAA-AAAA-000000000000    /Volumes/mountedImageTwo

The question is actually "How to remove...".

It can be ejected by using the device path

diskutil eject /dev/disk4

Substitute disk4 for whichever disk is offensive.

Mount them through Disk Utility and you will see them in Finder where you can just unmount them by clicking on the eject icon in the sidebar next to their names (in Finder) and they will disappear.