Systems and methods include a plug activated mechanical isolation device that controls fluid flow inside a tubular in a wellbore. The device includes a sleeve for coupling to the tubular, and the sleeve includes an internal bore and port for fluid flow therethrough. A channel element is positioned in the internal bore and includes an internal channel and an orifice for fluid flow between the internal channel and internal bore. The channel element is attached to the sleeve via a breakable attachment portion, and the orifice is aligned with at least one port of the sleeve. The channel element is slidable within the sleeve, upon breakage of the breakable attachment portion with a force, to move the orifice out of alignment with the port of the sleeve so that a portion of the channel element covers the port of the sleeve to block fluid flow through the port.