Final answer:
Jessie prevented Evan from selling lemonade by drinking the rest of his lemonade and offering her own, but he refused.
Step-by-step explanation:
Jessie prevented Evan from selling lemonade on the day she went to the beach with Megan by drinking the rest of his lemonade and offering her own, but he refused. By doing this, Jessie ensured that Evan couldn't make any more lemonade to sell.