The purge mechanism is two pieces. A rigid piece that kinds looks like a pizza with slices out of it, and then a covering flexible flapper valve. The flapper seals against the rigid bit until you exhale through your nose, then the flapper moves out of the way, allowing the water and air to flow out.
I've never looked closely at a Mako mask, but in most cases the bit that sticks up is the part of the flapper that pushes through the rigid part to hold the two pieces together. You might be able to trim off a little bit, but if you cut off too much the flapper will fall off then the purge will become the proverbial screen door in a submarine.
It may also me that the designer provided some sort of bit to help guide the water into the purge, in which case it could probably be trimmed - but you need to look closely.
Not to sound too snarky, but why have a purge mask at all? Clearing the mask (without a purge) is a basic skill that all divers should master the first night of open water class. Purge valves really are nothing more than Dumbo's magic feather - you don't need it to clear.
In addition, my experience is that purge masks are never as comfortable a non-purge masks. Also, the purge assembly is a failure point in that the flapper can get stuck open, or the entire purge assembly can pop out at an inconvenient time.
Non-purge masks are (generally) less expensive, more reliable, and more comfortable. My suggestion is to try one.
