Can an employer deny two weeks' notice?

No, an employer cannot deny two weeks' notice. Two weeks' notice is not a legal requirement but is generally just a show of courtesy to your employer. If your employer denied your notice by telling you that you cannot come in anymore, you may be entitled to involuntary termination without cause. That depends on your specific situation.

If your employer denies your notice by saying that you cannot leave, you simply should just continue working for those two weeks, as you intended. After those two weeks, you just leave the company and do not go back. That was the purpose of your notice, and employers must accept that.