Deleting lines in QGIS

Question

I have a layer with several shapes and from here I want to delete the lines. I tryed Toggle editing and Select features by polygon but when closing the polygon, it dissapears. Also I tried to delete all the nodes that I don't want by selecting them with the Node Tool buton, but nothing happened. What do you suggest me to do?

This is my example:

Answer 1

Your shapefile contains a multipart single feature as mentioned by @HDunn. In order to delete separate polygons, you will need to convert this to singleparts by using the Multipart to Singleparts tool:

Vector > Geometry Tools > Multipart to Singleparts.

I ran this and saved the output as "Free space v2.shp". Now you can edit and delete those pesky polygons!

I noticed there's quite a few of those dangles in your layer. A good method may be to use GRASS's v.clean tool with the rmdangle option to remove them automatically. You can access this from:

Processing Toolbox > GRASS commands > Vector (v.) > v.clean*

I used the following parameters (note the fairly high threshold):

Before being cleaned:

After being cleaned:

If there are any more polygons you don't want, you can remove them manually as there should be a lot less of them now.

Answer 2

First Make sure that you have enabled editing on the correct layer. Then use the Node Tool to select the nodes you want to delete (right click and drag to create a rectangle and select the nodes) Press delete and you are done, don't forget to save your edits.