How to delete geol codes from system picklists?

I've been off work for a year on Mat leave and I've come back to find whoever looked after my company's HBSI in a bit of a mess!!!!

The person has left the company now so I can't ask what they've done!

Basically they've added a whole bunch of geol codes to the whole system that I can't seem to delete from the home page before I click on a project. I'm creating new projects up North and its annoying I have a massive list of London Basin geology codes already in the project picklist!

Any ideas on how to delete them from the system picklist? The delete button is greyed out when I've tried.