I would handle it one of two ways. The first way would be to tell them that in your home you respect the life of all living beings so you do not allow any animals to be eaten in your home. If they feel so strongly about eating animals then you will have to go to someone elses home for Thanksgiving diner.
If you are willing to take a stronger stand, you can mention what I wrote above but instead of talking about going to someone elses home, just mention that you are going to have Thanksgiving diner at your home without serving "meat". You could then mention that you would love to have them eat with you but you are not going to give up your respect for animals by allowing animals to be eaten in your home. I would go on to say that if they can not respect you and your house rules then you will miss them at diner.