If the Box is offline
Disconnect the power to the box for a few minutes and the locking arm should release the cable.
If you have locked the charging cable permanently
Hold the touch button above the light strip on the Easee box until it turns green
Log in to the wireless network created by the Easee charging box
Open the web browser from your phone and go to 192.168.4.1 and enter the pin
Find the cable lock option and turn off permanent locking
If you have NOT locked the charging cable permanently
It is probably the locking arm that tightens against the cable and makes it secure.
Push the cable HARD into the charger
Press the touch button on the charger for 1-2 seconds while rocking the cable and pulling / pulling out the cable
Repeat a few times and finally the locking arm comes loose and you get the cable out. Exercise the locking arm a little up and down for a few minutes and the problem should not return.
Here is a quick video on how to loosen a stuck Easee cable: