Read this guide if you wanna hard reset your Nokia N97 device.

Steps “How to Hard Reset Nokia N97″

1. Back-up your files and contacts to a memory card.
2. Make sure the battery is fully charged
3. Turn off your Nokia N97
4. Press the green dial key, star key (*) and 3 key (3) all together
5. Press the power button for 1 minute to switch on the n97. Do not let go of the three keys until “formatting” is displayed.
6. wait for a couple minutes till formatting is done.

Now your Nokia n97 has back to default setting.
Or here is the other way.

1. Turn off phone
2. Take out SIM Card (only so you dont have to deal with the network err when resetting)
3. hookup USB so the charging begins
4. Turn on the phone
5. Once the phone comes up DO NOT LET IT LOCK it will not unlock if you do
6. Open the slider and just dial a few numbers
7. Wait however long you need for the dialpad to come up.
8. Use the dialpad to punch in *#7370#
9. Profit
10. Repeat shortly when it bricks itself again.

Folks from blog-n97.Fr has published an interesting comparison video of Nokia C6 and Nokia N97. It worth to check it out