Multi-site WordPress WooCommerce

Hi very strange to me, but this is the first time I used WordPress Multi-site. I have a wordpress with Woo-commerce installed multi-site, it has three sites, if I add one user as customer to one, the same customer can login to the other site with the same username and password. How could i avoid that, tried few plugins, but they all are very hard to work with and some make no sense. Any code ideas? Thanks in advance.