How do I stop 5v on the I2C port of a pro mini so I can use 3.3v devices?
Whoever decided it would be a good idea to default to using the Arduino internal pull-up resistors on I2C needs taking out back and giving a good kicking. It is so wrong on so many levels.

Yes, if you disable the internal pull-up resistors you can safely use 3.3v devices. The simplest way to do this is to turn them off again as soon as you have done Wire.begin(); by writing 0 to the pins with digitalWrite().