The Border Agency ordered squatters to leave their makeshift homes in Hendon last month.
Dozens set up camp on a disused football ground, claiming they had nowhere else to go.
They were offered flights back home to Romania.
Watch the report from Rags Martel:
More top news
Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.