Follow these instructions:
1. Write this down on paper:
csrutil disable
Note that there is a "space character" between the two phrases.
2. Boot to the recovery partition:
a. power down, all the way off
b. press the power on button
c. IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears
d. select the icon for the recovery partition with the pointer and hit return.
The Mac will now boot to the recovery partition.
3. When you get to the utilities screen, open Terminal
a. enter "csrutil disable" into the window -- then hit return
b. you will have to enter your administrative password.
c. terminal should tell you that SIP is now disabled.
4. REBOOT to your regular account.
5. NOW try to empty the trash.
BUT A WARNING:
Do you have something in the trash that is "OS-related", that is supposed to be part of the OS?
If you delete it, the OS may not work properly.
Suggestion:
Move everything in the trash to a folder on the desktop (call it temp trash or something).
Then move things into the trash ONE AT A TIME, and try to delete them.
What is the filename that "won't delete"?