Without knowing which version of ACL you have, it is difficult to give exact instructions, however look up Markers / Positions in the installed manual available under Help, there you will find more information about markers and how to delete them.
with audiolab 2016 right click on the marker you wish to delete. Scroll down until you see the track marker, then click it. Scroll up and you'll see that Delete is available. Click it and the marker will be removed.
also, if you haven't 'quite' clicked on it, the delete option won't be available - if this happens try clicking again - you should see the option to 'delete marker' just above 'delete all markers'