On ERAS, you have to check a box (one for COMLEX, one for USMLE) if you want them to report the scores with your application. You will also have to pay $50 each (COMLEX, or USMLE). If you are an MD student, you pay $50 for USMLE. If you are a DO student only reporting your COMLEX, you pay $50 for COMLEX. If you want to report both your USMLE and COMLEX scores, it's $100. (it's a one time fee and subsequent reporting when you get updates/step 2 scores ... won't cost you anything)
If you take the USMLE and don't want to report it, you don't have to report it. Now if a program ask you if you took the USMLE, then it's up to you (and your morals/ethics) whether you want to tell them the truth or not.
You also don't have to report COMLEX scores to MD programs too. Once you see ERAS, you can pick and choose what to report to which individual programs (which letters go where, which personal statement goes where, which program will get your COMLEX, USMLE, or both scores, etc)