I don't mean to nitpick, but I don't think it is possible (or makes sense) "not to have a default template", as your subject requests. But I know what you mean: you want to get back to the factory settings.
If you really have set a custom template as your default,
o Go to File | Templates > | Manage... .
o In the Template Manager, click the Action Menu icon at top right.
o Go to Reset Default Template > | Text document.
Note that this option appears only if you have set a custom template as your default. If not, note that some settings, such as default fonts and font sizes, can be applied through Tools | Options... . It may be that resetting these is all you need to do.
I trust this helps.
Brian Barker