Legal Question in Landlord & Tenant Law in California

It wasn't clear whether you already brought the problem to the attention of the building management, but if you didn't, you may want to start there. You could let them know the frequency of people using the roof, and remind them of the owner's and/or management's liability for someone falling, being drunk, tripping, vandalism, suicide, kids falling, etc., as well as the noise issue you are experiencing. Management could give a reminder notice to all tenants that the roof is not to be used or accessed, and post signs (or better signs if they are already posted).

If management doesn't do anything, or believes they are not allowed to lock the roof access, try contacting the county or city building department or possibly even the fire department to see if they can tell you what the rule is for roof egress. My understanding is that, typically, on an "unoccupied roof," which is what yours sounds like, access CAN be locked from the interior, allowing access to maintenance personnel or others who need access to any equipment on the roof. I am not aware of any law that limits access to roofs, but again, the building department or fire department may know of a rule that prohibits people from being on a roof that has no guardrails, etc.