How can I restrict jira agile labels to pre defined values

[Nic Brough -Adaptavist-]

There's no function for this - one of the main reasons for using labels is that users have free range to add anything.

There is a simple way to do it on JIRA though - create a custom field (a multi-select list), populate it with your pre-defined values and offer that to the users instead of (or as well as) labels.