@NicolaG10 Items queued to Queue Processors are pushed to Kafka, not database table.
In order to clear the queues, you can try removing the partition directories for your Queue Processor, which are created on the app filesystem. These partitions can be located in the 'kafka-data' directory on each app-node that supports 'Stream' service. For example, kafka-data can be located inside the tomcat installation directory for a Pega app running on tomcat.
Prior to removing the affected partitions from each node separately, ensure the app-servers are stopped to prevent replication between the Stream nodes. Otherwise the Stream service will repopulate the nodes with partition data soon as you remove it from any node within the cluster.