The 3.3v power supply going into the “BARO/TEMP” (as marked on Enviro pHAT) is made reasonably obvious by the decoupling capacitor that it passes through. If you carefully cut the tiny trace between the decoupling capacitor and adjacent pin ( toward the bottom edge of the pHAT, away from the text ) then power to the IC will be cut.
BEWARE! however that these types of IC have such low power requirements that they sometimes run quite happily parasitically. That’s probably less likely if you have multiple devices on the bus, but it’s possible that cutting the power trace wont be enough to prevent it from running.
The i2c Data and Clock traces are on the top right side of the IC as you’re looking from the top of Enviro pHAT with the text facing the right way up.
The thicker blue trace in the image is the 3.3v power trace on bottom side of Enviro pHAT, which passes up through a via (turning red to indicate top side) and then through the decoupling capacitor into the power supply pins of the IC.
If you could somehow connect the SDO pin on the Barometer to ground - it’s the bottom right pin on the IC, connected by a very small trace to the adjacent VDDIO pin on its left- then the i2C address of the BMP280 would change to 0x76.