@Oscar van Zelst​ apart from using configuration settings to restrict access, you can also use an init script to disable connections on a cluster. The script is:

%scala
val initScriptContent = s"""
 |#!/bin/bash
 |cat > /databricks/common/conf/disable_jdbc_odbc.conf << EOL
 |{
 |  databricks.daemon.driver.startJDBC = false
 |}
 |EOL
""".stripMargin
dbutils.fs.put("dbfs:/databricks/init_scripts/disable_jdbc_odbc.conf",initScriptContent, true)