Looking for suggestions: How to stop Marketo from automatically adding lead scores of the former and new lead record when merged?
You cannot.
What you CAN do is find a merge solution which allows you to determine the rules for merging and do it that way instead.
RingLead, Openprise, etc.
Add an Integer field, call it explicitly Winning Score. Copy the score (using Change Data Value) you want to keep to this field. Then merge the leads, and finally Change Data Value back to {{lead.Winning Score}}.