The only sure way to avoid random printing, in your case, is to keep printer turned off when not using it. Wireless devices fall into the IoT pitfall of security. The security is only as good as the manufacturer put into their network logic. If someone was able to hack your wifi network then they could use any device connected to it.. Your best bet is to use a strong wifi network password to prevent unauthorized use. Wireless devices are a nice convenience but they do put us at a certain level of risk.