I think there are a few things to consider here: yes you can 'spot tone' in the sense that you can focus on strengthening and firming specific muscle groups, but that's a separate issue to getting rid of the fat. And many exercises that are supposed to help don't do an awful lot.
But the good news is that there's lots you can do that will help. Please bear with me if I get a bit anatomical here!
With the abs, there are 3 groups of muscle - the straight down the middle muscle (rectus abdominis) which goes from the breastbone to the pubic bone - that's the one with the striations or 'washboard' you see on someone with very little fat in this area.
Then the obliques are the ones that run diagonally on either side - their job is to help the straight muscle and they are also involved in twisting or turning. The third muscle is the transversus which sits underneath the obliques and acts as a sort of girdle. It's not involved in any movement but it's crucial to posture (pulling the tummy in). It works particularly in conjunction with the back muscles, which is why it's important to hold it firm when making movements that could put undue pressure on the back, such as lifting heavy weights.
When you understand about the 3 types of abs muscle, you realise that not all abdominal toning exercises are the same. The kind of 'plank' exercises that strengthen the transversus and work on core stability do serve a purpose, but they don't work the rectus abdominis or the obliques as effectively as, say, crunches, where there is up-down and diagonal movement (eg opposite elbow to opposite knee).
Another common issue is people not pulling in their abs adequately when performing crunches or curls, so as they curl their body up the abs are being pushed outwards. (NB: tightening your abs doesn't mean holding your breath, as I'm always telling my husband in the gym!) You can't tell this from looking, as if you have fat there (which most people have) then that will bulge up - it has nowhere else to go. But if you give your belly a poke while in the crunch position then with a bit of luck what you should feel beneath whatever flab you might have is something firm and flat underneath it. If instead you feel a hard, domed muscle, as they say 'you're not doing it right'. A good exercise teacher would always explain this.
As regards the fat that accumulates on the tummy, it varies from person to person - you might find it's the first or the last place that your body holds onto fat. You can't spot-tone the fat away (that's the role of the 5:2) but you CAN tone the muscle that's there, so that when the fat is burned off your firm, toned midriff will be revealed in all its glory! (Although you would probably need a very low body fat percentage and you'd need to train like an athlete if you seriously wanted washboards!)
Another thing to consider is that a completely flat tummy all the time would mean holding the transversus taut all the time - which may not be comfortable or feasible - the tummy muscles are deliberately flexible to allow the abdomen to expand for example after a big meal, or in pregnancy. Holding your tummy in can also inhibit proper breathing in that it can impede the movement of the diaphragm. (I used to wonder why I always ran out of breath when singing, and then realised I had become so obsessed with holding my tummy muscles in I just wasn't allowing my lungs to expand and fill properly!)
Sorry for the long post, hope it's interesting. I used to teach exercise classes for a living, back in the day.
