the only real way to stop crab grass is to either spray it with a "non-selective herbicide" which will kill anything it touches. so be careful. or to put down a pre-emergence herbicide. it is to late for that one now though. it has to be put down before the crab grass germinates. which is late winter, early spring. have actually seen people put it down while snow is still on the ground. round up is the best non-selective herbicide i have ever used. if you have just massive patches of the stuff you can rent a sod cutter. cut out the crab grass, put in top soil, replant kentucky bluegrass seed. course i wouldn't be surprised if your top soil has crab grass seed also.
now thats out of the way. yes its crab grass. the "preventive" is going to do jack shit now. its what i said above, a pre-emergence. it has to be put down before the crab grass germinates. once its started only way i have seen to get rid of it is manually, or with the non-selective herbicide. also crab grass is a lot more hearty then your good grass. which in this area is usually a kentucky blue/fescue mix. oh if you use the sod cutter idea. it wouldnt be a bad idea after you have put top soil down to spray well with a non-selective herbicide. then wait how ever long it says to plant.