I wanted to know that How to block unknown browsers in
.htaccess? There is a Directive that can be used in
BrowserMatchNoCase but I do not know how to work with it.
You could create a whitelist.
However, this is a pointless endevour. While a lot of scripts and scanners, etc... will sometimes have a fake or custom created UA (user-agent), lots of the scripts these days will just mimic known UA strings, which renders just about every possible blacklist or whitelist approach pointless.