Use CRXDE/CRXDE Lite, find the templates that you have defined, delete them.
To remove predefined templates (Blog, Form Template) remove this paths in crxde:
To remove your templates:
1) Create page with template to remove (to check template path)
2) Check template path in crxde: /content/your_app/your_page/jcr:content in property cq:template
3) Remove template path (and temporary page)
I would acctually, when I think about it, use another solution. I would restrict the user/group that are to use the system from using that template or I would configure the templates I use to allow just a named set of templates using allowChildren/allowParents/allowedPaths. See http://dev.day.com/docs/en/cq/current/developing/templates.html.
To remove the predefined templates, blog and form template, bulan_'s solution works but it goes against the best practice of never touching anything in the /libs folder. This is also the case of Ove's solution.
Does anyone have a different solution to restrict these predefined templates?
Well it may not qualify as removing them, but one way to acheive this is to put a cq:allowedTemplates property on the top node of you site heirarchy. So for example if you site is located below /content/mysite and all of your templates were located in /apps/mysite/template/ you could add a property to /content/mysite/jcr:content named cq:allowedTemplates and set it equal to /apps/mysite/templates/*. That would prevent any templates other than your custom templates from showing up. See http://dev.day.com/docs/en/cq/current/developing/templates.html#Template Availability for a more detailed review of all the factors that can influence template availabitlity. You can see the impact of this setting at play by looking at the differences when creating a new page in the various Geomettrix demo sites (most of which have the cq:allowedTemplates set at the top level).
A less prefereable solution would be to deny read access to templates in question - but that could cause other potential problems.
Thank you, orotas. This is the correct solution to the original issue. The regex needed to be adjusted, but this worked great.
So, to recap:
Add a "cq:allowedTemplates" String property to "/content/mysite/jcr:content" and set it to "/apps/mysite/templates/.*"