Once you have activated Beast Mode, there is no way to turn it off until either the promotional period has ended, or you have placed the maximum number of eligible Beast Mode bets.
If you don't want to take the Beast Mode boosted price, you can remove the promotion off your bets in your bet slip by selecting another promo type (Pay Back Tokens for example).
Does Beast Mode have maximum stake amounts?
How can I place a larger stake than Beast Mode allows me to?