List<Contact> cont = new List<Contact>();
Map<String, inscor__Customer_Quote__c> cqMap = new Map<String, inscor__Customer_Quote__c>();
Map<String, inscor__Sales_Order__c> soMap = new Map<String, inscor__Sales_Order__c>();
Map<String, inscor__RFQ__c> rfqMap = new Map<String, inscor__RFQ__c>();
Map<String, inscor__Purchase_Order__c> poMap = new Map<String, inscor__Purchase_Order__c>();
Map<String, inscor__Repair_Order__c> roMap = new Map<String, inscor__Repair_Order__c>();
for (inscor__Customer_Quote__c cq : [SELECT Id, inscor__Contact__c FROM inscor__Customer_Quote__c]) {
    cqMap.put(cq.inscor__Contact__c, cq);
}

for (inscor__Sales_Order__c so : [SELECT Id, inscor__Contact__c FROM inscor__Sales_Order__c]) {
    soMap.put(so.inscor__Contact__c, so);
}

for (inscor__RFQ__c rfq : [SELECT Id, inscor__Contact__c FROM inscor__RFQ__c]) {
    rfqMap.put(rfq.inscor__Contact__c, rfq);
}

for (inscor__Purchase_Order__c po : [SELECT Id, inscor__Contact__c FROM inscor__Purchase_Order__c]) {
    poMap.put(po.inscor__Contact__c, po);
}

for (inscor__Repair_Order__c ro : [SELECT Id, inscor__Contact__c FROM inscor__Repair_Order__c]) {
    roMap.put(ro.inscor__Contact__c, ro);
}

for (Contact c : [SELECT Id FROM Contact WHERE inscor__External_Reference_ID__c = null]) {
    if (!cqMap.containsKey(c.Id) && !soMap.containsKey(c.Id) && !rfqMap.containsKey(c.Id) &&
        !poMap.containsKey(c.Id) && !roMap.containsKey(c.Id)) {
        cont.add(c);
    }
}

delete cont;