Create a hook to alter option values

Immediate use case for this is to allow permissioning on Case Activity Types (which are option value records).

You need to know the option_value.value of the activity type you want to remove / alter.

Usage examples:
// for case activities (e.g. New Activity drop-down in Manage Case)
function mymodule_civicrm_optionValues( &$values, $name ) {
if ( $name == 'case_activity_type' )

{ unset( $values[30] ); }

}

// for regular activities (e.g. new activity and activity type dropdowns)
function mymodule_civicrm_optionValues( &$values, $name ) {
if ( $name == 'activity_type' )

{ unset( $values[1] ); }

}