Hi there from Southern Catalonia, Spain. This fly has been a problem here for years, and the problem is getting worse.
The
local remedy is to suspend bottles of an attractant on every five
trees - with holes to let the little beggars in. Unfortunately, here they also spray the area using a small plane - which does little or nothing.
We pick our olives for oil early, and there are usually worms inside the olive, but if we leave them out in our trailer overnight, then the worms exit the fruit and crawl away.
We still make a good oil with low acidity (our last batch was 0.4º) but we cannot use this fruit for table olives which is a shame.
However there is always next year - good luck.