As the easement owner you have what is called the "dominant" estate and the land over which the easement runs is the "serviant" estate.
The serviant estate owner can use their land over which the easement runs in any way that does not interefere with the easement. The serviant estate owner cannot interfere with the reasonable use of the easement.
If the atv use is interfering with your use, then you can enjoin (stop) that inteference. You can also force the serviant estate owner to contribute to the maintenance of the easement do to their use.
You would have to file a declaratory judgment
and injunction action seeking what contribution to maintenance th serviant estate owner must pay and to stop unreasonable use of the easement.
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