Can your cheating spouse ever be faithful again? I think so, but it depends on the individual. You really can make a generalization with this question, there are too many different types of people out there. I will do my best to give examples of whether or not it could work again.
First and foremost, you need to figure out if your spouse cheated one time or multiple times. People are susceptible to temptation and can sometimes stray once without realizing the full repercussions of the situation. This to me means there was a will to stay faithful but they temporarily lost track of that goal. This is important because it establishes that they were not deliberately out on the prowl with no regard for your feelings, they were just caught up in a time of weakness. This type of person can be faithful again, but only if they realize to the full extent what they have done. They need to realize how they have hurt you and that it will never be alright that they have done what they've done. I do not think that this person should be given back the trust they were given before right away, they need to earn it back through proof of fidelity and counseling.
The second type of person I would like to discuss is the serial cheater. This type of person does not care for your feelings in the least. They will cheat with anyone who comes their way. They have you at home as a security blanket for when they have no other. They are not to be trusted and you should not expect them to change. This person has purposely gone out looking for someone to cheat with. Unlike the first person they don't care if they hurt anyone. All that they can think of is satisfying their sexual desires that will change and worsen and never be satisfied. This person will only bring you heart ache and disappointment, not to mention sexually transmitted diseases. Distance yourself from them. You can not change them and trying is only a waste of your time.
So there is your answer. I know it's not an easy thing to accept when your cheated on, but if you deal with the situation it will get better sooner than later. You will have to choose whether or not to forgive them or keep them. Thank you for your time, I hope I was a help to you.