You can send the neighbor a letter stating that unless he/she immediately removes the impediments to your right of way, and ceases further interference, that you will have no choice but to sue for damages for the invasion of your property rights. The letter will put the neighbor on notice that you believe the construction of a fence is unlawful.
That way, if and when you actually sue, the neighbor won't be able to claim ignorance and that provides you with a potential claim for punitive damages, which could cover your legal expenses, if you have to hire a lawyer to sue in your behalf.
Send the letter certified mail return receipt requested, so that there's proof the letter was received.
If you need a link to a reputable lawyer referral service, please let me know, and I'll be happy to provide further info.
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