I agree with 80smum to a certain extent, that the nursery should be supporting you in your efforts to put a stop to the biting, but I do not agree that the problem is wholly the responsibility of the nursery. Nurseries are there to provide care and education to young chiuldren, not to provide parenting skills or act as parents to the children in their care.
Saying that, however, the nursery should be supporting you and your ds in tackling this negative behaviour. The nursery saying that they will exclude your ds on the basis that he bites is a bit of a cop out in my opinion, but if the ratios in Spain are so poorly organised, you have to expect that the support is not going to be the best. Show me a toddler who hasn't experimented with biting. It just doesn't happen! Or if it does, that child is in the minority!
Saying no when your ds bites and simply watching him will allow you and the nursery to determine when the biting is going to take place. The nursery should be doing observations on the children in their care, so will be able to work out a pattern. Does he bite because he wants a toy, is it because he is near a certain child, a sudden change in routine???
If the nursery isn't prepared to look out for the reasons as to why the biting is happening, they cannot exclude your ds, as they have cannot have an action plan in place to deal with it.
Maybe meeting with the manager/key worker and asking them to observe your ds over a period of time will help you all identify the reasons for the biting and then help you come to some sort of conclusion as to how to deal with it.
By the way, I certainly wouldn't bite a child back. I don't like to be bitten so try to imagine it from a child's point of view. They are too young to understand and as someone else rightly said (can't remember who as I've been waffling for so long!) it gives the signal to the child that biting is acceptable.