There should be a comma after silly. The word inspite is two words. And delete the article 'a' before common.
I think this whole thing is silly, to be honest. However, I do understand that you guys share one thing in common: enmity towards Billy. I do respect your guys' decisions that he shall not join your parties in the future, join me when he is present, nor invite me because of the probability of him joining; but I do, in spite of all, think we all can find common ground, which doesn't feature blocking somebody.