Hi Sasha.
When creating a template you can choose both how to scope it and also if it be used as default. You can scope it to workspaces or projects. That will allow workspace admins or project admins respectively to be able to delete it later on.
However, users within these projects (Editors or Viewers) will be able to apply that template , but not to delete it.
My take on what you're asking is how can you ensure that a template is 'uneditable' and available for your users for use. If that is what you meant by 'LOCK' then it should be achieved if you scope your template at the proper level you'd need to allow for administration. For all other users it should be 'fixed' and so for any purpose it's 'locked'.
Please let us know if that answered your question and helped you resolve.
You can learn more on template scoping at:
Work Item Templates | CA Agile Central Help
Thanks,
Sagi