My
Decathlon bike has Tektro IO discs front and rear. Here's how I adjust them - although you might still end up with very slight rubbing of pads to disc as I guess cheap discs may not be absolutely perfectly true in the first place....
1) Slacken off the cable adjusters at the lever and at the brake end of the cable so the lever will probably come back and nearly touch the handlebars if you apply the brake.
2) Slacken off the 2 allen bolts that hold the caliper to the bike, so the caliper can move side to side.
3) Tighten up the allen bolt in the back of the static caliper, so the static pad is pushed hard against the disk until the wheel wont turn at all.
4) Now tighten up the 2 bolts that hold the caliper to the bike.
5) Now slacken off the allen bolt on the back of the static part of the caliper so that the static pad moves away from the wheel, allowing the wheel to turn. Keep undoing that allen bolt in small increments and spinning the wheel each time to see if it is spinning freely. As soon as it is spinning freely, your brakes are as good as they are going to get.
6) If there is too much cable travel in the brake lever, you may need to take up some play with the 2 cable adjusters.
7) Job done. Now CHECK ALL THE BOLTS THAT YOU HAVE LOOSENED AND RE-TIGHTENED to ensure that they are tight.
8) Try the brake by riding the bike in a safe place.
9) If, like me, you are a bit paranoid, check the bolts again just to be sure! And that's it. Best of luck