It is the latest incident to happen in the Dunscroft Community Garden which was opened in 2010 in memory of Liam Maughan, who lost his life aged just 18, serving with third battalion, The Rifles.
Staff at the East Doncaster Development Trust , which looks after the garden next to its base on Station Road, arrived on Monday July 10 to find the plaque carrying Liam's name and his regimental badge had been prised off a bench to which was fixed, and severely damaged.
The tools used to remove it also caused damage to the plaque, which trust trustee Bill Morrison says is now unusable.
The incident comes around a month after a tree which was planted at a centrepiece of the garden, by Liam's parents, was pulled from the ground.
There are concerns that the tree will not survive the incident.
Mr Morrison said: "We are going to have to get a new plaque now because vandals have had a go at the current one.
"It had been pulled off, thrown on the floor and damaged. It is all scratched now. We can't put it back up.
"It is clear to anyone with any sense that this is a memorial to a man who was killed in action - people are well aware of what this garden's all about.
"The people who are doing this vandalism are absolute morons.
"The garden is visible from the road, it is not hidden away. This has gone beyond vigilance."
He urged anyone who had seen anything to call the police.
The earlier incident, which Mr Morrison says was the latest in an ongoing problem of vandalism at the site, saw the seven year old memorial tree pulled from the ground. It has been placed back into the ground, but there are concerns it may die as a result of what happened.
It has been proposed that it is replaced with a rosebush if it dies.
Former Hatfield Visual Arts College Liam lost his life while on patrol. He was shot dead by a sniper while on a rooftop in the town of Sangin in March 2010.