There are ways to hide the Upload Button using CSS or JavaScript but this is not recommended. I suggest creating a custom site content type that is associated with your InfoPath Form.
This article provides a step-by-step process of Publishing your InfoPath Form to a Content Type and then associating the content type with a document library: http://www.appvity.com/blogs/post/2013/06/16/How-to-configure-and-publish-InfoPath-to-SharePoint-2013.aspx
Microsoft provides the information here: https://support.office.com/en-us/article/Create-a-SharePoint-library-or-site-content-type-for-your-form-template-ae137cab-bad4-42f0-8105-34f6b1430a70#bm4
Once you have published your InfoPath Form to the library, you can remove the content types except for the one associated with your InfoPath Form:
Go to the library from which you want to remove the content type.
Click the Library tab, and then click Library Settings.
Under Content Types, click the name of the content type you want to remove.
Under Settings, click Delete this content type.
When you are asked if you are sure you want to delete this content type, click OK.
Using this method, users will only have the ability to upload a document using the InfoPath form.