This page explains what user talk namespace templates are, and how to use them within Wikipedia. You can click on the tabs at the top of this page for detailed information about how to use these templates, as well as how they have been designed.
User talk templates are placed on users' talk pages to advise a user against actions that disrupt Wikipedia, to advise editors of common mistakes, or to place a standard boilerplate note at the top of a page. They are designed to convey a standardized message. The purpose of user warnings is to guide good-faith testers and dissuade bad-faith vandals or editors engaging in disruptive editing. You should check that the user has made harmful or disruptive edits before issuing a warning, and that they have not already been warned for the same action by another editor. The user must be given a chance to see, and react to, each warning given.
User blocks serve to prevent further disruption, or to protect Wikipedia, rather than to punish bad users. If a user stopped vandalizing some time ago, and their edit histories don't suggest a pattern of chronic vandalism, there's no need to warn or block them at all; a welcome template for new users might help future visits. Conversely, if a user is in the midst of an obviously bad-faith vandalism spree, there's no need for them to be warned before an administrator temporarily blocks them. You should ensure that you are familiar with the content of templates before issuing them. If the tone or content of a template isn't appropriate, don't use the template — just say it normally.