How do I stop it from creating this plate when starting a new part?

If that feature is in every new part, it is in your start part.  You need to remove it from the start part.

It is very likely a feature in your start part. 
In case you don't know where your start part is located, check the following config option:

template_solidpart

If the new part is sheetmetal:
template_sheetmetalpart

Check this link for more details:
Configuration Options for Object Templates (ptc.com)