Legal Question in Real Estate Law in California

There is no way to answer this question without seeing the title documents for both parcels.

The general rule is that the holder of an access easement (Including someone who acquires use of the property accessed by the easement via a lease) may make use of it in ways that the original parties might reasonably have foreseen or intended. For example, it's foreseeable that someone using the access easement might have a breakdown and need to leave his broken-down car parked on the easement for a day or two. In some unusual easement situations, it might be OK for the easement holder or his guests to park on the easement on a regular basis. However, what you are describing sounds as though it would fall into the category of "overburden", which means the easement is being used impermissibly or excessively. See the 1958 California case of Keeler v. Haky, 160 Cal.App.2nd 471, holding that an easement to pass through an alley did not include the right to park vehicles long-term, but indicating that occasional or temporary parking might be permitted. As to what to do about it, I'd certainly recommend negotiation as a better remedy than a lawsuit, but you might want to retain a local lawyer with easement experience to write an initial letter asking the neighbor to cease.