Is there an example how to configure UdfAccess setting in firebird.conf?

Well, there's one right there in the firebird.conf, but perhaps it isn't obvious enough. Here are the basic settings ('None' to disallow UDFs completely and 'Full' to allow them anywhere) which you probably understood yourself:

UdfAccess = None
UdfAccess = Full

And here is that tricky Restrict setting:

UdfAccess = Restrict C:\some\directory

For multiple directories, use something like this:

UdfAccess = Restrict C:\some\directory;C:\some\other\directory

For Linux users:

UdfAccess = Restrict /some/directory

In the default setting 'Restrict UDF', 'UDF' is a directory relative to root directory of Firebird installation.