I would put a sticker over the OBD connector warning of a remap even after informing the garage.

My Focus ST went in for a service at a Ford dealer and had the map overwritten even after I told them not to do so.
They failed to pass on the info to the technician who promplty followed the service procedure as normal.

Luckily, I had the ability myself to reprogram the ecu so didnt cause me a problem !