You will need to delete the device using the connect website. Please see the link below for step by step instructions.
https://forums.garmin.com/showthread.php?134948-Delete-a-device
If the above steps did not resolve your issue please call our support number listed below. If necessary, a case can be opened for you so we can look into the problem further and determine why you are not able to delete devices and/or are seeing duplicate products.
By Phone
U.S.: 913-397-8200
U.S. Toll-free: 800-800-1020
Canada: 866-429-9296
Support Hours
Mon–Thurs: 8 a.m.–6 p.m.
Fri: 8 a.m.–5 p.m.
Central Time (Closed Holidays)
There are UK numbers on the Garmin web pages, just look under support. They've always been quick to respond I found although I haven't used them for a while.