now that mosaico has 3 base templates for people to choose from - we don't want our users choosing any of those and only want them to use the custom template we created. Is there something I can do?
We did it by just removing those template files from the server (I couldn't find a less brute-force way to do it). In wordpress they are here:
I think there is nothing that allow to hide base templates for now.
Personally, I would like to see a functionality that allows hide selectively the usable templates from a list of available templates. Perhaps a new featured request could be opened on https://github.com/veda-consulting/uk.co.vedaconsulting.mosaico on this direction.