If one spouse violates the separation agreement, the other spouse can seek to have it enforced. How the enforcement is done depends upon whether the agreement is simply a private contract between the couple or whether it became part of a court order.
If the agreement is a private contract, the spouse who violated the contract can be sued for breach of contract in civil court. The harmed spouse can ask for money damages and/or request that the contract be specifically enforced. If the agreement is part of a court order, the spouse who violated it can be brought before a judge who may decide to hold him or her in contempt – which could include payment of attorney fees and jail.