Then you can demand they remove it.
The law on this matter is clear. When a person owns property, they are free to do what they like with that property (so long is the use is consistent with federal/state/local regulation).
That includes preventing others from improper use of the property
What you describe, you are a property owner that has an easement in favor of your neighbor. Not uncommon. But an easement is a limited property right. Basically, as the property owner, you own the property and you are free to use the property as you like (again, subject to regulations)...and you can enforce this right against others.
The neighbor has an easement...he has the right to use the property SUBJECT TO THE EASEMENT.
SO, for example, if the easement was to allow construction
of a treefort? Then he would be fine...but I doubt that the easement provides for that.
More likely it is a transit easement..it allows travel across the easement from a public road to their property.
They can do this...all day long. You can not stop it.
But the easement does NOT give them the right to build a structure on the property.
YOu can demand they remove. IF they refuse, you can remove it yourself, and bill them for any costs you incur
let me know if you have more questions, happy to help if I can