You cannot make a truly alcohol free beer. The best commercial brewers can do is get it down to about .5% ABV. At home, most report that about 1.5% ABV can be achieved with boiloff. However, it is reported to have a severely negative impact on flavor. Having tried this, I would advise you not to waste your time.
The bitterness is concentrated, not reduced. However, since the malt is also concentrated, it makes the bittering sometimes (not always) appear less.