Comment 43 for bug 1872527

I came across the same issue with a guest distro.
I finally solved it by editing the XML File of the "Display Spice" section in the VM settings in virt-manager.

Open the details window of the affected VM in virt-manager.
Navigate to the section "Display Spice".
Click on the Tab "XML" (XML editing has to be enabled in the virt-manager settings).
Display Spice" section can be editted manually.
Add "<clipboard copypaste="no"/>" to the XML. It should look like this:
 <graphics type="spice" autoport="yes">
   <listen type="address"/>
   <clipboard copypaste="no"/>
 </graphics>

Source: https://unix.stackexchange.com/questions/341881/virt-manager-copy-and-paste-is-possible-to-disable-it