"以契约约束" can be translated into English as "bind by contract" or "be constrained by a contract".
Here are some example sentences to illustrate the usage:
1、 Both parties shall be bound by contract to fulfill their respective obligations.
（双方均应以契约约束，履行各自义务。）
2、 The agreement binds the two companies by contract to cooperate closely.
（该协议以契约约束两家公司紧密合作。）
3、 We must ensure that all transactions are conducted in a manner that is bound by contract to protect the interests of all parties involved.
（我们必须确保所有交易都以契约约束的方式进行，以保护所有相关方的利益。）