A property owner may not block access to an easement area.
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While this person seems to be acting under permissions of the neighbor, he has no right to block the easement. If there are other owners who are blocked, the group of you can bring an action for interference with the easement.
It may be that your local community has a mediation service. El Cajon or the County of San Diego. That might be less expensive than bringing the required action to enjoin the defendant from parking his boat on the easement.
Seek out an experienced real estate lawyer in your area, and have them examine your option in San Diego County.
Joseph C. Melino
San Jose, CA
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I agree with all counsel above and would add that not only is his blocking the easement a violation of all of your mutual and "non exclusive" rights, this behavior may constitue an actionable "nuisance claim". Is there a recorded easement agreement? There may be more rights...and potential remedies...than mentioned above. There may also be violations of local ordinances due to, for example, interference with emergency acess. You may wish to explore a free or limited consultation to flesh out more details. Good luck.