Is it Islamically permissible to destroy the internet cable at the workplace to stay away from pornographic websites, knowing that internet usage is optional?

Your destruction of an internet line that you do not own is impermissible. You should have asked the service provider to block it from your device. Your destruction of the line is not justified by your fear of falling into vice, for the internet has permissible and beneficial uses that the company has the right to benefit from. To absolve yourself, you must repair the line or bear the cost of repair, unless the company waives its right. You must also repent to Allah for violating the rights of others.