There is a * user agent that can be used to prevent all bots that honor robots.txt from crawling a site. Specific user agents can then be granted additional access afterward.
If so, are you willing to share an example robots.txt file? What bots would be allowed? Google, Bing, ...? Those?
Any robots.txt I might suggest wouldn't necessarily be appropriate for anyone else. It's a policy that site stakeholders need to decide for themselves I'd say.