A barrier for the control of people has a stand (10, 11) supporting a housing (12) for a flexible band (14) extensible from the housing through a slot (15) in the side wall thereof. The spool and band are removable through the open upper end of the housing (12) and a tensioning cassette (16) for the spool is engaged with the housing and is drivingly coupled to the spool (13). A spring tensioner (29) acts between an outer wall (26) of the cassette and a drive shaft (30) for the spool. Tension may be applied to the spring (29) by freeing the cassette (16) from the housing (10) while leaving the drive shaft (30) connected to the spool (13) and then rotating the cassette, relative to the housing.