I don't claim to know but I'd have thought you'd need something spirit based such as white spirit ? But I guess that might depend on the type of ink. Certainly I'd try that but I take no responsibility if the plastic melts into a puddle on contact ;-)
home.howstuffworks.com/how-to-remove-stamp-pa
d-ballpoint-pen-i...
- not sure if it helps (or indeed if the link works) but googled in 'how to remove ink stains from plastic and got a good few suggestions
Ballpoint ink is usually easy to remove with alcohol as a solvent. If you have methylated spirit, neat vodka or even de-icer - should work and not harm the lens.