A simple non-invasive fix is to add friction externally with a rubber or silicone band.

Users commonly stretch a silicone wristband, hair elastic, or similar rubber band so it partly overlaps the focus ring and another fixed part of the lens (or sits under the lens hood and below the focus ring). This adds drag and makes the ring harder to turn accidentally. The same idea can help with an aperture ring that gets bumped too easily.

There are also purpose-made products for this, such as lens anti-creep bands, but a generic silicone or rubber band often works just as well.

Tips:

• Use a band wide enough to create steady friction.
• Adjust the stretch and position until the ring has the resistance you want.
• Don’t place it so tightly that it binds or interferes with normal operation.
• Avoid anything adhesive or anything that could leave residue on the lens.

This is an easy, reversible solution and is safer than trying to alter the lens mechanics internally.