A pump dispenser comprising: an actuator head having a dispensing channel communicating with a nozzle outlet; a closure body having a top facing, at least one engagement feature extending axially upward from the top facing, and a recess formed in the top facing, said recess having a cylindrical sidewall with a plurality of axially-aligned stoppers positioned along or around the cylindrical sidewall and an annular ledge defining an inlet port at a bottom end of the recess, said ledge including a cylindrical abutment extending axially upward within the recess; a resilient bellows having a cylindrical outlet positioned along a deformable top wall and a cylindrical enclosure extending downward from the deformable top wall, said cylindrical outlet including or connecting to a plurality of axial outlet flanges, extending below the deformable top wall, with a web formed proximate a terminal end of each axial outlet flange; wherein the actuator head is biased upward by the resilient bellows, with the cylindrical outlet of the bellows coupled to the dispensing channel of the actuator head; wherein the resilient bellows couples to the closure body so as to form a pumping chamber defined by the deformable top wall, the cylindrical enclosure, and the top facing; and characterized in that: the pump dispenser further comprises an internal plug positioned within the annular recess, said internal plug having an annular disk with at least one axially-aligned stopping flange positioned at or around a periphery of said disk and a plurality of axially-aligned engagement flanges positioned toward a center of said disk; said web supports a centrally positioned engagement nub; and the inlet port is selectively held sealed by the annular disk, said annular disk being secured by at least one of (i) the engagement flanges coupling to the engagement nub and (ii) the at least one stopping flange abutting the terminal end of at least one of the axial outlet flanges. an actuator head having a dispensing channel communicating with a nozzle outlet; a closure body having a top facing, at least one engagement feature extending axially upward from the top facing, and a recess formed in the top facing, said recess having a cylindrical sidewall with a plurality of axially-aligned stoppers positioned along or around the cylindrical sidewall and an annular ledge defining an inlet port at a bottom end of the recess, said ledge including a cylindrical abutment extending axially upward within the recess; a resilient bellows having a cylindrical outlet positioned along a deformable top wall and a cylindrical enclosure extending downward from the deformable top wall, said cylindrical outlet including or connecting to a plurality of axial outlet flanges, extending below the deformable top wall, with a web formed proximate a terminal end of each axial outlet flange; wherein the actuator head is biased upward by the resilient bellows, with the cylindrical outlet of the bellows coupled to the dispensing channel of the actuator head; wherein the resilient bellows couples to the closure body so as to form a pumping chamber defined by the deformable top wall, the cylindrical enclosure, and the top facing; and characterized in that: the pump dispenser further comprises an internal plug positioned within the annular recess, said internal plug having an annular disk with at least one axially-aligned stopping flange positioned at or around a periphery of said disk and a plurality of axially-aligned engagement flanges positioned toward a center of said disk; said web supports a centrally positioned engagement nub; and the inlet port is selectively held sealed by the annular disk, said annular disk being secured by at least one of (i) the engagement flanges coupling to the engagement nub and (ii) the at least one stopping flange abutting the terminal end of at least one of the axial outlet flanges. the pump dispenser further comprises an internal plug positioned within the annular recess, said internal plug having an annular disk with at least one axially-aligned stopping flange positioned at or around a periphery of said disk and a plurality of axially-aligned engagement flanges positioned toward a center of said disk; said web supports a centrally positioned engagement nub; and the inlet port is selectively held sealed by the annular disk, said annular disk being secured by at least one of (i) the engagement flanges coupling to the engagement nub and (ii) the at least one stopping flange abutting the terminal end of at least one of the axial outlet flanges.