In the event of a dispute, can the client change their mind and refuse arbitration, or withdraw from it?

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No. The arbitration clause is binding for both parties from the moment the contract is signed, and neither party can unilaterally withdraw from it thereafter.

The fact that, despite the existence of an arbitration clause, the other party does not agree to arbitration proceedings, is of little significance. The arbitrator will conduct the proceedings. A judgment will be rendered even if the defendant, despite being properly notified, refused to participate in the case or did not actively take part in another way.