Should I attend a job interview after I've already accepted another offer?
Once you've accepted a job offer, you are supposed to inform other places that you've applied that you would like to withdraw from consideration.

If they still want to invite you over to give a talk knowing that you can not be considered for the position, go ahead. But, you must tell them.

To do otherwise would be a serious breach of ethics. You do not want to gain a reputation as someone who engages in unethical behavior (don't assume they won't find out).