The Mont Ubé Dolmen, a Neolithic passage grave, was originally built about 3,500 BC and 2,800 BC.
Societe Jersiaise posted the photos on Saturday and soon "had a very brave admission" from those responsible.
Jersey Police said it had received a formal complaint and would be investigating.
Archaeologist Robert Waterhouse said dozens had come forward to offer help in removing the paint.
He said: "It was absolutely wonderful to find so many messages of support; we are rather touched by it.
"Mont Ubé is one of my favourite Dolmens and it was upsetting to find it damaged but I was encouraged to find a lot of people share my view that it is a wonderful place."
He said they could not take people up on the offer as the stones would need specialist treatment as the paint was "stuck on and difficult to remove".
Source: Societe Jersiaise
Vandals who wrote "kill" and "die" on an ancient monument in Jersey apologised after a heritage group posted pictures on Facebook.