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T-Pain ﻿ain't down for shots to the face -- even if it's just a beach ball -- and his fans definitely know that now after he abruptly pulled the plug on a weekend concert.

We got this video from his Sunday set at the BBYO (B'Nai B'Rith Youth Org.) Convention in Denver, and you see him rapping "Bartender" as the beach ball comes flying through the air, bouncing off the side of his dome!

It definitely looked funny, but T-Pain wasn't in a playful mood. He immediately beelined for the exit. Fans at the concert tell us T-Pain was only about 30 minutes into his set when he bounced.

Tension was building because witnesses tell us T-Pain had already warned the crowd he was leaving if they threw another ball onstage. We're told T-Pain's people tried to get him to go back on ... but he refused.