How can I disable the iMessage app temporarily on my iPhone
You cannot actually disable iMessage as an app, as it is integrated within your Messages. iMessage was introduced by Apple as an alternative texting feature between Apple devices (Mac, iPod, iPhone, iPad) over WiFi rather than through your carrier which would require a texting (SMS/MMS) plan.

You can however disable iMessages in your settings -> Messages -> toggle off iMessage.

However this would just cause messages to come in via SMS instead of iMessage, unless the sender can only send an iMessage (such as a Mac, iPod or iPad without carrier), in which case the sender won't be able to send to you at all.

My recommendation is turn of the phone, turn on airport, set a lock password, fingerprint, or hide your phone better. :)