to fix upon (one-sidedly); to (ignore someone's position and) arbitrarily decide something is the case
to scold; to take (a person) to task
彼を犯人と決めつけた。
They decided he was the culprit.
勝手に決めつけるな。
Don't jump to conclusions.
上司は部下の失敗だと一方的に決めつけている。
The boss is unilaterally deciding it was the subordinate's fault.