Can a TV sitcom like TBBT be pulled off the air prematurely?

Yes, Any network could change their decision and cancel a show after renewing it or (more likely) order a small number of episodes. For example The Brink starring Jack Black. HBO renewed the show for season 2 then reversed the decision

From this

HBO Cancels The Brink

The network has reversed its Season 2 renewal

HBO has cancelled The Brink after its first season, changing its tune on the freshman series after initially renewing it. The Tim Robbins and Jack Black comedy was halfway through its debut run when word first broke that HBO had picked up the show for Season 2.

While it is unclear what changed the cable network's mind in the months since, HBO did release the following statement:

"After evaluating our schedule and our programming needs, we unfortunately decided we cannot give The Brink the attention it deserves for a second season. We are proud of the first season and wish everyone involved in this show the very best."

But it's unlikely something like that happens to TBBT since it's the highest rated CBS show

Sure it can. If one of the main stars say or do something stupid, any show can get canned. For example professional Blowhard Donald Trump racist remarks resulted in Univision and NBC from dropping his shows. http://starcasm.net/archives/320385

“At NBC, respect and dignity for all people are cornerstones of our values,” begins a statement issued by the network earlier today. “Due to the recent derogatory statements by Donald Trump regarding immigrants, NBC Universal is ending its business relationship with Mr. Trump.”

The statement reveals what the end of their relationship with Trump means. “To that end, the annual Miss USA and Miss Universe Pageants, which are part of a joint venture between NBC and Trump, will no longer air on NBC,” the network says. “In addition, as Mr. Trump has already indicated, he will not be participating in The Apprentice on NBC.”

The Food Network also dropped Paula Deen cooking show after her racist remarks.

Max Headroom was cancelled partway into its second season, when it was put up against Miami Vice, on one network, and Dallas, on another, and basically died in the ratings due to it's time slot. The final episode makes ironic reference to this in a speech the character gives:

"We will fight them on the beaches of Miami," said Max with the fervor of Winston Churchill. "We will fight them on the sidewalks of Dallas. . . . And if the ratings system lasts for thousands of years, men will turn and say, this was Max Headroom's finest hour!"

Ironically, shows that have one of the main actors die, such as Dan Blocker on Bonanza, or Peter Duel in Alias Smith and Jones, or Redd Foxx in The Royal Family, etc., tend to trundle on until the end of the season, trying to survive without the main character, but tank. This is usually due to contractual agreements and threats of lawsuits.

Also, it's rather rare for a cable network to cancel a series they paid for; the cost structure is much different for network television shows, which tend to not do their own production, but the networks are also the ones with the biggest contractual disputes, since they know the show can't go on, but they in insist on the show going on, in the hope that the production company will eat the loss, instead of them eating the loss.