BOSTON — The state Attorney General's Office has reached an agreement with Bay State Gas Company that reduces the company's proposed rate increases for its 321,000 gas customers by more than $11 million and prohibits the firm from imposing more rate increases before March 2021.The company, which also does business as Columbia Gas of Massachusetts, serves 321,000 customers in over 65 cities and towns in and around Springfield, Lawrence and Brockton.The settlement, filed with the Department of Public Utilities last week, resolves a proceeding before state regulators where Columbia Gas sought to increase its net distribution revenue by $24.7 million.If the agreement is approved by the DPU, the new distribution rates will take effect Nov. 1.