How do I replace a string value with a NULL in PySpark?

You can combine when clause with NULL literal and types casting as follows:

from pyspark.sql.functions import when, lit, col

df = sc.parallelize([(1, "foo"), (2, "bar")]).toDF(["x", "y"])

def replace(column, value):
    return when(column != value, column).otherwise(lit(None))

df.withColumn("y", replace(col("y"), "bar")).show()
## +---+----+
## |  x|   y|
## +---+----+
## |  1| foo|
## |  2|null|
## +---+----+

It doesn’t introduce BatchPythonEvaluation and because of that should be significantly more efficient than using an UDF.