There is no way to pin barman package to a certain version.

[ghost]

Barman package gets updated whenever yum update or yum cron update is run.

In some circumstances it is beneficial to have an opportunity to lock the package to some version, say, the newest version has a bug or there are some breaking changes that need investigation.

[shamil]

this is what I do:

class { '::barman':
   ensure => '2.8-1.pgdg16.04+2'
}