When browsing the backed up file Macrium Reflect assigns drive letters in my system like "File Backup (F: ) ", "File Backup (L: ) " and so on, See image below.
They look like attached hard drives.
They remain even after restarting my computer, several times.
Trying to access these returns error message "F: not accessible".
I am not able to erase or detach these "hard drives"
How do I get rid of these?
Macrium soon have occupied all possible drive letters

When you are done browsing an attached drive, be sure to detach it.  That should prevent what you are seeing.

However to get rid of drives that you are seeing you can use your OS Disk Management option.  Go to administrative tools, select Computer Management, and then select Disk Management.  You should see a list of drives near the top.  You can right click on one of the drives in the list and there are a lot of options - including delete.