The answer is - this is not possible. 

I have contacted the helpdesk and in short find their answer below.

The work around is - delete the erroneous entries, and create new ones if you have to with the correct values; using the list view this is quite easy to do

• filter on the person / task / project you need to remove
• select the check boxes of the entries you wish to delete
• go to the action button and select "delete" from the drop down menu.

Hope this helps some of you.