Who was the first "cancelled" actor?

Yes, probably - I don't recall ever hearing of an earlier example. Certainly he must have been the first that would have been huge, shocking news at the time. 

My first thought was "Fatty" Arbuckle. Despite being acquitted, his career and reputation as a comedy actor suffered after the trial.

If Booth had been captured alive and incarcerated, no doubt he would have had a following of female admirers.

I thought of Fatty Arbuckle first but if you really want to get technical,
the winner probably is:

en.wikipedia.org/wiki/Genesius_of_Rome

One day Genesius, leader of a theatrical troupe in Rome, was performing before the Roman Emperor Diocletian. Intending to expose Christian religious rites to ridicule by his audience, he pretended to receive the Sacrament of Baptism.[3]

As the play continued, however, Genesius suddenly lay on the stage as if very ill. Two performers asked what was wrong. Genesius said he felt as if a weight was on his chest and he wanted it removed. Two actors, dressed as a priest and exorcist, were called on stage. He said he had had a vision of angels bearing a book listing all of his sins. The "priest" asked, "My child, why did you send for me?" Genesius said he could still see angels and asked to be baptized right there. The "priest" did so. Enraged, Diocletian had him arrested and sent to Plautia, prefect of the praetorium, to be tortured. Despite his agonies, Genesius persisted in his faith, and he was finally ordered to be beheaded.