How do I blacklist this cell phone?

Is the handset yours or did u buy it off someone? It could be a stolen phone, & the owner has reported it stolen. All networks would have it blacklisted!!

You'll have to contact the service provider who blacklisted the phone. Most won't do this though as the phone would have been blacklisted for being lost or stolen.

Check the attached links,
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Galaxy S5 Blacklist Repair
Unblock Un blacklist Galaxy S5 Mobile G900T
How to repair Blacklisted IMEI on Samsung Galaxy S4 S5 Note 4 NOTE 2 Note...
http://www.imeigurus.com/product/samsung-s5-bad-imei-fix-remote-imei-repair-service

Presume your only resource is to notify your Cell Phome service so your phone should blacklisted.
Bad news is that serious phone hackers can by-pass this blacklisting or even sell it to international gangs which "re-export" stolen and hacked devices
So your only hope is that it was stolen by some amateur thief.

The IMEI code is permanent and will identify your phone when it talks to the cell phone company.

Free check Blacklist Mobile by IMEI / ESN / MEID You can check your IMEI number is Blacklisted in AT&T, Verizon, T-Mobile, Telus, Sprint...Blacklist IMEI is lost, stolen or ineligible (Bad, Barred, Blocked, Financed, Under Contract) mobile phone. Always check blacklist before buy, sell or get phone.Supported:+ Private database (more than 1 million imei)+ USA: AT&T, Sprint, Verizon, T-Mobile, Cricket, Metro PCS+ Canada: Telus+ Sweden: Telenor+ Japan: Softbank, AU KDDITo find your IMEI number, type *#06# on the phone keyboard, or is located in the battery compartment.Please enter the 15 digits, without any other character.
https://imeidata.net/blacklist/check

Then it is probably a stolen phone. I suggest you refer to wherever you got the phone.

a blacklisted phone is a stolen phone,and the owner reports it to the service provider to block it after confirming the requesting client is the owner they verify and block so it may work in onother country other than where it was stolen from,but its against the law to aid things of that sort,but if you change the imei #,serial # it can work in your country

This cell phone has been reported lost or stolen and can no longer be used.