I've discovered that I can solve this using a simpler method - it seems that an antijoin is possible as a parameter to the join method, but the Spark Scaladoc does not describe it:
import org.apache.spark.sql.functions._
val df1 = Seq(("Hampstead", "London"),
("Spui", "Amsterdam"),
("Chittagong", "Chennai")).toDF("location", "city")
val df2 = Seq(("London"),("Amsterdam"), ("New York")).toDF("cities")
df1.join(df2, df1("city") === df2("cities"), "leftanti").show
Results in:
+----------+-------+
| location| city|
+----------+-------+
|Chittagong|Chennai|
+----------+-------+
P.S. thanks for the pointer to the duplicate - duly marked as such