"cordon it off" is a correct and usable phrase in written English.
You can use it as a way of referring to the action of separating or isolating something. For example, "The police officers cordoned off the area to keep the public safe."
Hundreds of motorists allegedly drove past the bomb after cones cordoning it off were removed.
When soldiers raided a home, he said, they first cordoned it off with Humvees.
The police had cordoned it off, so you could not get out even if you wanted to.
For days, pilgrims had poured into Har Ki Pauri until Wednesday morning, when the police cordoned it off to prepare for the arrival of the holy men.
Maintenance workers quickly cordoned it off with yellow caution tape, then put garbage cans underneath to catch the water that started pouring through about 10 minutes later.
In late 2005, insurgent activity along the route became so intense that the brigade in charge of the province at the time cordoned it off with barbed wire and abandoned it.
I couldn't get into the city of Arsal, a mountain enclave whose name is Aramaic for "God's throne," because the Lebanese Army has cordoned it off to outsiders.
Detectives, uniformed officers and members of the medical examiner's staff, who converged on the scene late yesterday and cordoned it off to passers-by, were seen removing what appeared to be a body wrapped in a yellow plastic sheet or tarpaulin.
The farm remained sealed off after a team of more than 100 police officers cordoned it off along with two properties in Cardiff and Monmouthshire early on Monday.
Last month, residents took matters into their own hands by getting the road tarmacked but safety inspectors deemed it dangerous and cordoned it off again.
