QCAD shows only entities that are present in a drawing and are not on invisible layers. Whatever you see on your screen is an entity that exists in your drawing. It is not possible to make a cleanup algorithm since QCAD cannot know which entities you intend to keep and which entities you would want to delete. For QCAD, every entity in your drawing is equally relevant and legitimate.
You can usually select entities (to delete them) by clicking on them. In some cases, the entity might be hard to select (e.g. a line with length zero or an arc or circle with a radius of zero). In this case, use the region selection:
- move mouse cursor to a position slightly to the left and above the entity
- press down the left mouse button
- move mouse cursor to the right and below the entity
- let go of left mouse button