You can prevent IDM from starting the download using two ways,
The first way is by specifying file types that IDM should automatically grip, so you can exclude the certain file type, for instance "MP3" or "MP4".
To do that configuration, you have to click on "Downloads" menu in the IDM's main GUI, and then select "Options", a new small window will appear, choose "File types" tab from it. Now in the first text input box you can add or remove the file types you want
The second way is by specifying certain sites that IDM shouldn't automatically download from them. You can find an input text box below the text box in the previous way which you can use for that purpose.
for example you can add something like this:
dropbox.com
to prevent any automatic download from Dropbox site
Note:
- In the first way, you can enforce IDM to download from a specific link when its targets file type is not listed on the list of automatic grabbing types. to do that you should first click on "Keys..." button which is located in the "General" tab in the "Option" window, and from the window that will appear, make a tick in the second choice which will allow you specify a certain key that could be held while clicking on the desired link in order to enforce the IDM to start the Download
