Mutual interference between indoor adjacent wireless networks is becoming an important issue due to reducing the communication speed significantly. Controlling the isolation of wireless networks by using frequency selective surfaces (FSSs) can be an efficient solution for such interference problems. Therefore, a switchable bandstop FSS design is presented in this work for unlicensed 2.4 and 5.8 GHz ISM (Industrial, Scientific and Medical) bands. In order to achieve desired frequency switch performance, a new theory is proposed in this paper by using PIN diodes. According to the proposed theory, PIN diodes should be placed in positions where high current densities are observed or where charges are collected. By using the proposed theory, a new FSS geometry design was carried out to control the targeted ISM bands. According to our best knowledge, there is no similar FSS work in the literature with four different frequency responses for 2.4 and 5.8 GHz ISM bands.