He prevented a fight from happening
This sentence is a complex sentence, as it contains an independent clause ("an onlooking man prevented him") and a dependent clause ("he was about to lace them to the boy's feet"). The dependent clause cannot stand alone as a complete sentence.
My immortality prevented me from experiencing death.
"I did not let her" or "I did not allow her."
The closed system prevented the flys in.
I prevented my friend from doing a dangerous flip
His admonition prevented the little boy from getting in trouble.
My friends ethnic culture prevented him from eating certain foods.
It was the overweening confidence of the candidate that prevented her from acknowledging her weaknesses.
The hostile group of youths prevented me from entering your hostel.
My amnesia prevented me from remembering that I should not plagiarise sentences.
Katy's family had planned to leave for Ohio that day, but the snowstorm had prevented them from leaving.