Is it true that once one had worked with TCS he would not be allowed to work again in TCS for lifetime?

"How" is not the question The question is "why"

It's not true. If you are fired then you cannot be hired again but if you resign now and comeback to TCS after 5/7 years. They will hire you but you should have a good network in TCS

Yes , this is true as per current tcs policy, hope tcs will modify their policy and allow people to rejoin again!!

No i have seen recently a mail from colleague joined second time and leaving tcs for second time

Yeah true

Yes

Absolutely right, You cannot join TCS again once you leave.