You can't remove gratings from the list without actually deleting it from the database, but you can choose what proffered sizes you want for the grating element, sizes that will appear in front when accessing the roll-down menu, while the other ones will be in the "All" category, not visible by default but with the option to access them if needed.
In order to configure the proffered size, you need to open "Management Tools" and go to the tab " Proffered sizes". Here, select the languange - Standard - Grating. You can choose the preffered sizes indivudually for Standard grating and Variable grating. The configuration is made by checking the left box corresponding to a grating type.
This preffered sizes configuration is avaliable to other elements as well, like beams, bolts, plate, shear studs and welds.