1. Open and notorious
2. Actual and uninterrupted
3. Exclusive
4. Hostile and under a claim of right
5. For a period of 10 years.
I would simply interrupt the possession, by putting something like a park bench on the property, use the bench, and do so openly. This would bar the second, third, and likely fourth prong of adverse possession. You are free to write a certified letter to the other party stating this, but you don't have to--the claimant of adverse possession has to prove all five elements to get quite title.
Consult with a real estate attorney in your area if you have concerns about your particular situation.