As an agency user you have no limitations on cancelling shifts. You can cancel and edit allocated shifts when needed. This can be done on any of the allocation screens like this:

To do this in the onsite screen you would need to go to Plan, then Shift allocation (on-site). Once you have opened the shift allocation page for your site, all you need to do is click on a shift, it'll open up these options below where you can choose to cancel a shift:

You can also cancel a shift using the shift details option shown above, this will open up detailed information on the shift you selected and give you a red cancel button like this:

You also have the option to do this in the branch screen. To do this you would need to go to Plan, then Shift allocation (branch). Once you have opened the branch screen for your site, you will need to click on a shift/shifts here: