How can I delete a line from a scanned pdf

Bottom line is you can't do it with Acrobat alone. You need to use an image editor, like Photoshop, then create a new PDF from that edited image.

Most of the people you meet here are not working for Adobe, we are users of Adobe's technology (hence the "Community" title of the site). 

A scanned document is a picture of the original page. There is no way to edit that on a character or word, line or paragraph basis, unless you use an image editor application like Adobe Photoshop. If you have Photoshop, you can actually edit a page right from Acrobat's Edit tool in Photoshop by right-clicking and selecting to "Edit Using>Photoshop" (you will have to configure Photoshop as a touchup editor first in Acrobat's preferences). This does require that you know your way around Photoshop, but you should be able to remove a line - as long as it's a complete line. 

In order to make a scanned document more accesible, you would want to OCR it. This will add search capabilities to the document, but may also provide a way to edit the document. When OCRing a document, there are basically two output formats: The recognized text can be stored "behind" the original image, in which case you are back to square one and would need an image editor, or you can replace the image with text that is now set in the font from the scanned document. In this case, you should be able to remove a line by using the Edit Text tool. Depending on how the document is structured, it may not be a simple process, and you may have to move other stuff around to make things look good again. PDF is not a good format to make edits in, I like to call it a final file format. However, you can export your document to Word and then edit the Word document, and then save as PDF again. How well this works depends again on the document, and if there are other parts in your document that may not look good in Word, one way to do this is to edit the document, convert to PDF and then just use the one page you've modified and replace the page in the original document with this modified page