Privatization
The Subsidiary DLC lets you float a subsidiary through an IPO. Privatization is the same road traveled the other way: turning a public company back into a private subsidiary.

Taking a company private
Buy the company outright
You must hold 100% of its shares, bought from the Stock Market. Anything less and the option will not appear.
Open Financial Actions
Press F11 or open the Financial Actions screen, then select the target company.
Privatize
Privatization now appears in the list of financial actions. Click [Privatize This Company] to carry it out.