Here is a break down on how you will be able to complete this task:

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1. In the Domains section of WHM's Tweak Settings interface (WHM >> Home >> Server Configuration >> Tweak Settings), set the Prevent cPanel users from creating specific domains setting to On.

2. Create a file called: /var/cpanel/commondomains

3. Add each domain extension (TLD) on a separate line, and do not prepend or append a dot to each domain extension (TLD). For example:

cat /var/cpanel/commondomains
.tk
.ml
.igg.biz
.ibiz.cc

4. Save the file
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The change is effective immediately, therefore no further action required. Then when you attempt to create a domain with any of those TLD, it would returns with a message:

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Account Creation Status: failed

(XID 73t9nu) The system cannot create the common domain "domain.tk". You must choose a different domain name.
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