1 Click on the point where you want all remaining material to be gone (but don't click on any particular track, that'll just select thatr track).
2 Hit 's' to split all tracks at that point.
3 Click and drag over the bits you want to delete (if they are not already selected)
4 Hit 'delete' to get rid of them
I think the easiest way is to probably make a time selection that includes everything you want to keep, then use the action "Crop project to selection".