I am not really sure if you want to keep the images catalogs and delete only the entries, or if you can delete the images catalogs themselves.
For the second case DLTIMGCLG KEEP(*NO) will do the job.
For the first case, as a brut force program, you can use RMVIMGCLGE IMGCLGIDX(index) KEEP(*NO) with index value going from 1 to 256 and monitoring CPFBC30 and CPFBC31 to avoid errors when there is no entry for the index.
If you have several images catalogs, look at *IMGCLG objects in QUSRSYS. There should be one for each of images catalogs that you see with WRKIMGCLG. So you can browse through those objects and do what is needed for each in a CL program.
Make sure that the image catalog is no longer loaded into a virtual device.