Bring in Pathet Lao units south of the blocking line and occupy cities and towns. The Royal Laotians don't have enough units to protect the cities AND set up a line across the country.
How does this help? Occupying with face down Pathet Lao doesn't hurt anything.
Occupying the cities with face up Pathet Lao just makes the country go Allied (which invites scores of Marines and swarms of planes into Laos).