Please keep in mind that your call blocker stores three (3) phone lists:

Incoming call List -- The device holds up to 100 numbers in this list. These are the most recent 100 numbers that called your phone line.  You can browse (or navigate) this list by simply pressing the scroll buttons.  You can remove numbers from this list by pressing the REMOVE button while scrolling the numbers. 

Blacklist - This is the most important list in the device.  You can add (unwanted) numbers to this list by scrolling the incoming call list and pressing the BLACKLIST button.  Depending on the version, your call blocker can store from 1,000 to 1,500 numbers in this list.  You can remove numbers from this list by first pressing the BLACKLIST button, then the scroll buttons to find the number you want to remove, and then press the REMOVE button (just press and release within 1 second).  
WARNING:  If you press and hold the REMOVE button for more than 3 seconds, it will prompt for PASS, which is a way to protect your Blacklist.  The default password is 0000.  You will need to have a phone connected in series in order to enter this password, if you wish to delete your entire Blacklist at once.

OUT List - This is the list of outgoing phone calls you have made from your phone line.  This list is only available if you have the call blocker is installed in series (e.g. you have a phone connected to the PHONE port of the call blocker).  You call blocker holds up to 100 numbers in this list (most recent outbound calls).  You can remove numbers from this list by pressing the REMOVE button while scrolling the numbers. 

If after reading this, if you still have trouble deleting a number from your Blacklist, please contact support.