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When a technician observes unexpected foam accumulation during treatment, clamping the venous line is the appropriate action to take. This is crucial because foam can indicate problems with the dialysate or the patient's blood, such as air in the system, which poses safety risks. Clamping the venous line helps to stop blood flow and prevents further complications while the issue is assessed and addressed, ensuring patient safety.

The other options do not effectively address the immediate concern of foam accumulation. Notifying the patient’s family may be important for communication but does not solve the technical problem at hand. Increasing pump speed might worsen the issue by circulating more foam into the system. Changing the dialyzer could be a necessary step later but is not the immediate response needed upon noticing foam. Thus, clamping the venous line effectively prioritizes patient safety and allows for further evaluation and corrective measures to be implemented appropriately.