C172 flight instructor observed and reported another aircraft caused a critical ground conflict at a non towered airport.

Narrative

On Day 0 at XA47; I was an instructor in my plane at ZZZ [airport]. I was in the runup area with a student when I saw Aircraft Y also enter the runup area. I advised my student that I had seen that plane commit numerous unsafe actions before; including runway incursions. Aircraft Y completed a very quick runup and then immediately pulled forward; announcing over CTAF that 'Aircraft Y' was departing runway XX. They taxied at a very fast pace directly from runup and onto the runway; without stopping at the hold short bars to clear traffic on final. There were numerous planes making CTAF calls; including one on final. Aircraft Y then braked hard; stopped; announced on CTAF something to the effect of 'Actually holding there's landing traffic.' Aircraft Y was fully on the runway at this point though however; and the entire plane was past the hold short bars. Aircraft Y remained in place as another training aircraft came in for landing. While that aircraft likely should have gone around; they continued with a touch and go.After Aircraft Z touched down; Aircraft Y then stated they were going to 'line up and wait' again another action not to be done at uncontrolled fields. During this time; there were still other planes making radio calls; including another place that had been announcing they were on final. Aircraft Y pulled out and waited on the centerline of the runway for a few moments and then began departing. There was another aircraft though that was on very; very short final behind them and had to go around as well. Aircraft Y is a complete menace to ZZZ [airport]; and presumably any other airport they visit. They commit runway incursions routinely; forcing extremely unsafe situations. Their complete disregard for safety caused two near misses during this one incident that could have been significant. I have previously filed an additional report on this place and other instructors have as well.