With model 304 firmware for the BLI ON30, the only way to avoid the holiday announcement is to avoid executing the bell and horn actions that trigger these sounds. There is no CV or Analog Programming Option
to disable this feature.
The alternative is to upgrade your locomotive with model 303 firmware for the BLI ON30. Model 303 and model 304 firmware are identical except that model 303 does not have the holiday announcement feature.