For entire network, "negate cell" will help - because entire network includes every node registered in system. So to match everything except one printer, you should add only that printer to source and then select "negate cell".
However, if you wish to match all hosts in specific subnet except some host from this subnet, it will not help. In this case, you will need to use "script" column. For example, to exclude device with ID 100, script may looks like following:
OBJECT_ID != 100
Best regards,
Victor