Access Virtual Device Manager and remove them from here:

If that still proves difficult, you can try removing the VDC using Spectrum CLI
#1. Gather model handles of each VDC you wish to manually remove.
- go to Attributes tab of the VDC model
- look up "model_handle" and make a note of it
#2. Nav to <SPECROOT>/vnmsh and log into CLI
./connect
#3. Destroy the VDC models:
./destroy mh=<model_h_of_vdc>
#4. Repeat as needed until the VDCs are manually removed.