You definitely don't want to prune watermelons. According to [url=https://www.soilandhealth.org/01aglibrary/010137veg.roots/010137toc.html]Root Development of Vegetables Crops[/url], watermelons react badly to having their vines trimmed. Click on Watermelons on this link, and go to the very bottom of their section to learn a little more about that.
Your vine probably didn't stop growing because it was going upwards. I tried that last year, and although the vines hated it, turning sideways and trying to go back down, they didn't die until the plant was suffocated (RIP) by its pumpkin roommates.
I'm betting Jal_UT is right, that the problem is space. Dumb me is trying to grow 5 watermelons (3 Sugar Baby & 2 Fordhook) in the same 8' x 4' bed, and I will consider myself lucky if I get 5 watermelons from the lot. No matter how much compost tea I feed them, they are not happy campers
Live and learn. Next year . . . yeah . . . Next year is the year I do nothing stupid. You just wait and see