I always play Devil's advocate......challange their own boldness and logic by asking simple questions.
First off, it's easy to state that the basic claim "golf is bad for the environment" is the equivalent of claiming "teenagers drively hazardously fast." The statement is always offensive to somebody, never 100% accurate, but will be true for a number of the population of subjects. So....sure, some golf courses are bad. And once they hopefully agree that not ALL golf courses are bad for the environment (if you can't get past that simple agreement, walk away from the conversation), then start asking for specific examples of HOW golf is bad for the environment.
Even your average semi-avid golfer that truly loves the game and wants to stick up for it will have some simple valid points that can dispell this nay-sayer's claims. And even if you don't, keep asking them to get more specific, or accurate, or better ask them what their resource is for getting the information they may claim they have.
Mostly, statements like this are merely a perception or an ingrained, non-thinking doctrine. It's easy to LOOK at a golf course and say, man, there's no way they can make this place look like this without some crazy stuff that MUST be bad for the environment. So people will make their statements with little to no fact finding done in advance, just on perception.
Find out how "educated" your subject is, then follow-up with research of your own to support golf. There is plenty out there.
Common things people that claim this will use to support their claim and simple rebuttles:
"Golf courses use a ton of harmful chemicals." - Cite organic golf courses. Ask them about chemicals they use in their home, on their front lawn, or home cleaning chemicals they put right down the drain that go into sewer and storm drainage, unfiltered, that then eventually can reach creeks, rivers and wildlife there. Then ask how many houses could be put on the same land as that golf course and how much total chemicals those households would contribute to the environment.
"The area used by golf courses destroys natural habitat for animals." - Ask them to cite which animals the golf course displaced, and prove that these animals no longer still use the golf course as their home. Ask if the golf course WASN'T there what they think that area would be used for. Still natural and wild? Housing tract? Parking lot?
"Golf courses use too much water." - Ask them if they know how much water an average family household uses, especially on their lawns and gardens. Then ask them to try and do the math on which is more efficient. Ask them how much water their local water park uses. Or the local car wash. And ask them which amenitiy they would rather have.
We all know these statements to be bogus, and just knowing that the claim "Most golf courses AREN'T bad for the environment" is more accurate than their claim means that you don't even have to KNOW the answers to why they AREN'T, you just have to be able to point out to them the flaws in their arguement and let them do their own deductive reasoning.