OK, if you don't have a buy-out agreement within the partnership agreement - then you have limited options.
A partnership does not require a written agreement that includes a buy-out provision but it's smart if they do. If there is an agreement, then the partners are free to determine in that agreement their rights, including how and when and on what terms a partner can withdraw from a partnership.
If you do not have a written partnership agreement, then any partner can withdraw at any time under California partnership law. By withdrawing, a partner is entitled to be bought out. The first step is to get an “accounting” which is a financial snapshot. Then, the bought out partner is to be paid for his share.
However, you can't force a partner to relinquish their interest in the partnership - force them out.
Your only options are to come to agreement as to buying them out and if you can't dissolve the partnership, have an accounting, split the assets, and each move on.