It would actually be an SQL query, not a template modification, as it would involved changing the xf_user_follow table, and even then there would be nothing to stop them unfollowing.
However, you would need an add-on to do it for all new members.
I have no problem with them un-following. It just seems like its an unused feature that I would like to highlight better. Most people have no idea that you can follow other users. I've made improvements on my end but I think it would help if new users were following someone by default. Its not a major issue, just something that I would like to implement. All new members would follow one specific member by default (for example me or a team account).
Would this be a difficult add on to build?