When a site owner, or someone with the right permissions creates an alert template on a list, every alert created after that uses this template by default.
However, if a member wants to, he/she can still create his/her own template. In this case, the site owner wants to control what information is sent by e-mail, so it would be a bad idea if members can put anything they want in an alert.
My solution: as a site owner, create the template. Make a new permission level for this specific list, where members can not create alerts. Then (site owner) make an alert for the members (alert for sharepoint group).
This sort-of works, but is not an ideal solution, I would prefer it if members can still create alerts, but not choose or make their own template. Is this possible?
'Locking' templates for members
This is not supported with the current release of Alert Manager. However it is on our list of possible features to add to a future release although there is no planned release date as of yet.