You can make it such that nobody, except those with the Administer Jira permission, can create new team-managed projects.
Removing this permission does not delete existing team-managed projects. Users with the Administer Jira permission can always create team-managed projects, even if that group is removed from this permission. (If you'd like to prevent even admins themselves from creating new team-managed projects, please vote for https://jira.atlassian.com/browse/JSWCLOUD-21477 as suggested by @Vero Rivas
You can remove this permissions via the Global Permissions settings in the System settings (you will need to be a site admin or a Jira admin to do this). Here's some screenshots to guide you :)
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