The pages of your website are divided into sections: content blocks are also divided into columns and lines.
Delete a column
To delete a column, make sure that you are in Content mode.
Click on the column that you would like to delete then click on "Delete the column".
Delete a line
To delete a line, make sure that you are in Structure mode .
Click on the line that you would like to delete then click on "Delete the line".
If you would like to know how to add a column/line, have a look at our dedicated article here.