Block specific file types while uploading from SharePoint interface

One easy way:

We can block specific type of files from entering file extension into “Blocked File Types” from the SharePoint Central Admin Site -> Operations-> Security Configuration.

We can select specific web application for blocking or not allowed file types/extension list.
 
Other way:

We can also do programming and modify current Upload.aspx page, we can write simple JavaScript by checking file extension and content type when user is uploading any file and then we can give alert to user if its not valid file type or extension type for uploading into SharePoint.