Hey @gcheung! There isn't a built-in mechanism for opt-out policies within the Browser agent, but I think that's a great feature to be suggested! I'd include this in our Feature Ideas area to let other members of the community vote on it and add their voice to the conversation.
One option that some customers have found success with is putting the Browser script inside a conditional statement on the page that has logic that determines if the customer has opted in or not. This isn't officially supported by New Relic, but might be a good option for you.
Let us know your thoughts or if you have any questions!