If you get desperate, I've given my agent a custom $PATH that replaces the forbidden tools with shims that either call the correct tool, or at least tell it what to do differently.

~/agent-shims/mvn:

    #!/bin/bash
    echo "Usage of 'mvn' is forbidden. Use build.sh or run-tests.sh"

That way it is prevented from using the wrong tools, and can self-correct when it tries.