It is possible to withdraw paper after conference is held but paper still not published in ieee xplore.
The formal "point of no return" is the moment when you have submitted the copyright form. After this, you can still ask the conference chairs and the IEEE contact person to withdraw the paper. While you should not expect a positive reply, there's still a certain chance that they accommodate your request based on good will.
To improve your chances, you should reach out to them as soon as possible. Later in the publication process, the effort for performing the withdrawal may be greater, and you would also have wasted more time on their side to process your paper.