By "my guys" I assume you mean the tattoo artists that are providing tattoos and that they are providing you with a percentage of what they charge the customers. It sounds as though you have nothing in writing with the artists. You may have problems other than just customer complaints. They may be considered to be employees under federal and state law which would potentially subject you to liability for employment taxes such as FICA (social security) which should be withheld from employee's paychecks.
Whether they are contractors or employees is generally determined on a case by case basis and you should hire an employment attorney to help you figure this out. It sounds like you don't want liability for their actions and so you want them to be independent of your operations. Perhaps a lease between the artist and you that provides that they are paying rent in the amount of the %age and then a notice to customers that the artists are not employees or independent contractors of the establishment, but rather are independent businesses that rent space? This is just one idea and should not be construed as legal advice as I don't know your situation. You should consult with a WI attorney to help with this matter. The last thing you want is the IRS suing for past years of employment taxes....