"Some property rights routinely receive constitutional protection, such as water rights. For example, if land is changed from waterfront to inland property by the construction of a highway on the shoreline, the owners of the affected property are to be compensated for their loss of use of the waterfront."
Although this individual is not a government, the precendent set by the 'routinely granted' water rights easements means that the construction of this fence may be in abrogation of the landowners rights. I don't know this for sure, so I merely mention it in passing and will not argue this point strenously.
I will argue strenuously, however, that the pond may be condemned under Eminent Domain, and that the government would be just and legally permitted in so doing, provided they satisfied the 'takings' clause in the Fifth Amendment with an equitable compensation. I will also argue strenuously that condemnation under Eminent Domain is a morally-neutral tool that has a long history, and that its use was sanctioned by the Founding Fathers of the United States of America.
Kinda like how they do it in Zimbabwe?
Legal maybe, but I'd be interested in why you think it would be "just" to take this person's property.