Yes, you can reject a job offer even if you have signed the offer letter, as long as you have not officially joined them for first day of work.

Of course, they will not be happy and may blacklist you, so the decent thing to do would be to inform them early so that they can find a replacement asap.

In my years of work, I have been ghosted quite a number of times by candidates before, haha..The worst are those that only informed us on the day that they are supposed to start work.