War Machine -- BLACKLISTED FROM PORN ... After Alleged Attack at Porn Party

Ex-MMA star War Machine took a stab at a porn career after he was kicked out of the UFC -- but was quickly blacklisted after beating up several porn people ... so says XXX star Brooke Haven.

Haven tells TMZ Sports ... she invited Mr. Machine to her birthday party back in 2009 after they performed a sex scene together.

But when he got to the party, Haven says someone hit on the fighter's date -- and things got violent ... quick.

According to Haven, Machine didn't just ruin the party -- he ruined his shot at a porn career ... because after the "massacre," no one wanted to work with the guy again.