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Your surveyor is the first place to start.  I would assume that he is licensed by the state and knows state laws regarding easements.  And easement is a right to use another's land.  In most cases the uses are limited, such as an underground easement to provide utilities; having an easement does not mean the land it open to public use.  It is possible that the land in question belongs to the state, and YOU have an easement for its use (or the other way round).  If the city/county/state owns the land in question, you need to find out eactly what your rights are to its use, and limiting the use by others.
I am assuming that the kids driving their ATVs are not licensed, and that the ATVs are not either.  If the county does not allow you to fence in your property or will not enforce trespassing, you need to inform the county that they will be liable for damage to your property by these people, as well as liable for any injuries they sustain.  Seeing that the easement is more or less extensions to the sides of the road, it is likely that laws regarding driving can be enforced there as well.  Not sure about where you are, but here driving across open fields to avoid roads is illegal here.