1) Go to the little menu button on top right of screen and click on it

2) Go to My Account > choose tab "TOUR OPERATORS"

3) Simply deselect the following option "Make this tour operator visible in my inventory" for each company you don't want to see in list. 

4) You can also hide only specific tours per tour operator by opening up the tour operator and deselecting "Sell Tour" option.

And that is all! Good luck :)