They use anti bot technology, a program which tells them if a human or a program is playing the game.
Of course they can't catch everyone and scripters/developers try to act as human like and as undetectable as possible,
however the risk is always there to get banned.
They mainly focus on banning goldfarmers though.
I don't see reports as a big problem regarding bans, it's more to get caught by their anti bot program.
Since a few weeks, osbot supports the mirror future which has lowered the banrates significant.
For informations about the mirror future click here
Also if you browse through the section "Runescape Bans" you will find a lot of discussions regarding bans, how to lower the risk and more.
Good luck