Question:

Why is a document still merging after the Merge Fields document field has been removed from the Document Type in Vault?

Answer:

When a document has a document field that has a value and that document field is removed from the Document Type, the document field and value will still exist.

Thus, for the document field Merge Fields, if this field is on an existing document and the field was set to "Yes" and then the document field was removed from the document type, the document will still have the document field Merge Fields set to "Yes" and thus will continue to merge. 

To stop the document from merging, the document field Merge Fields will have to be re-added to the document type and the field on the specific document has to be changed to "No" and then remove the document field Merge Fields again from the document type. 

Related Documentation:

Vault Merge Fields Documentation: Enabling Merge Fields for Microsoft Word™ & Excel™