Is an exclusivity agreement enforceable by law?

While there are a number of different types of “exclusivity” agreements, I assume that you are referring to an agreement under which your client would agree to work exclusively with you or obtain your type of services exclusively from you.

If that’s the case, then such an agreement should be enforceable, although you need to be careful that this agreement doesn’t constitute a “non-competition” agreement, since those types of agreements can be problematic in certain states, especially California.