I worked with Sf21 and S2 cells and rarely had contamination. What type of cell medium do you use? Do you supply it with serum and antibiotics? I used penicillin.
The only option you have is to discard those plates and start again with a new vial of cells. But first check if the contamination is in your media: just put a plate with fresh medium and no cells in the incubator. Sometimes, even very experienced people get bacterial contamination in their cultures.