Hullo and thank you for your post.
I'll do the best I can to help you.
If I understand correctly: let's say you have one Keynote presentation with superfluous material (you probably have many from the way I read your post). You would need to open it and then as you run through the many, many "slides", you could hold down the Command Key, use the mouse to scroll through your presentation and left click on those you want to delete.
At the end result of this tiresome process, you will have selected all those you want to delete. Then use the Backspace Key to remove all of them.
Once deleted, run through your presentation to make sure that everything is as you want it, then and only then, save it.
May I offer a piece of advice to prevent this happening in the future?
Let's say you are starting a new presentation - we'll call it Project.1
When you get tired or are, in any case, only part way through, Save it (as Project.1).
Next time you open the presentation, before you do anything to it, Save it as Project.2 Work away on this, stop and save.
Then next day, you open Project.2 and before doing anything, Save it as Project.3 And so on.
One day, you will be finished and hopefully happy with things - Project.8 perhaps
You can then delete all the previous versions.
It's a mistake, in my view, to keep working on the original. Always create a new version, as above.
Please tell me if I've understood your post correctly and anything more you might want.
Ian