Here's an update which gives Garmin Support's answer and a possible workaround that builds on some tests I ran partly triggered the by the very useful ideas from JFHEATH and IKRUAS (many thanks guys).
First, the Garmin Support answer:
"I've had a further look into this and unfortunately it looks like the TripAdvisor mapping which causes this information to show up cannot be disabled."
HOWEVER, here is a possible workaround that, on a small amount of testing seems to work, but I'd be interested to know if it is really a workaround or if the effect is conicidental rather than causal.
1. Clearly, the Zumo XT2 cannot be pulling data frome elsewhere - everything else (phone Tread, other maps) is turned off - so the issue is located on the device.
2. I could not initially explain why, approaching a Waypoint from one direction, Tripadvisor popped at Destination arrival, but approaching the same location from a different direction Tripadvisor did not pop at Destination arrival.
3. After rereading the ideas from JFHEATH and IKRUAS it occured to me that the Destination (though phisically the same palce, same GPS coordinates) targetted in 2. above were in fact TWO separate destinations to the XT2. The FIRST INSTANCE was entered from Garmin Search (which I think uses the onboard Tripadvisor database) from home whilst the SECOND INSTANCE was entered from an arbirtary location on the road using the Garmin Address dialogue.
4. It led me to speculate that ANY Destination targetted through reference to the Garmin Search (Tripadvisor?) dialogue would trigger Tripadvisor to pop at Destination arrival whilst ANY Destination targetted "independently" via an Address (even if the same physical location on GPS coordinates) would not trigger Tripadvisor to pop at destination.
5. I deleted all Waypoints and Routes from the XT2 and, as a test, re-entered THREE of the waypoints via their address (not using Garmin Search or Garmin History).
6. This morning, I visited all three Waypoints and Tripadvisor DID NOT POP at any Destination Arrival.
7. My working assumption is that any Destination "harvested" from Garmin Search (Tripadvisor?) data has embedded metadata which causes Tripadvisor to trigger. Conversely, f the destinaton is entered "cleanly" as an address then no metadata is attached to it and Tripadvisor does not pop.
8. This is not conclusive proof of what's going on on the device, BUT for the 3 tests I've made it works empirically whether causally linked or only coincidentally.
9. I'll be adding a few more Waypoints using this method in the comming weeks and it will be interesting to see whether or not this is a valid workaround.
I'd be interested to know whether and XT2 owners out there can replicate this test and the results.