Be careful to not apply any force to the aperture ring. If you do, the mechanism can be damaged. Many iris mechanisms use a small diameter driving pin with fine screw threads to fasten the pin to the aperture ring. The inner end of the pin operates the iris. Its easily bent or broken.
Possibly, a tiny drop of penetrating oil like Liquid Wrench (oil + solvent) introduced between the threads from the top of the lens board will ease the removal of the lens, provided that the lens board and lens can be removed from the enlarger.
You can apply a thick wrapping or black cloth friction tape of the kind often used to improve the grips of hammers and other tools that require a secure grip to the barrel of the lens, but avoiding the aperture ring.
This might give you sufficient grip to remove the lens. Afterwards you can clean the tapes adhesive off the lens barrel with a tiny amount of naphtha in a small piece of rag.
