Oh wow, now I think I understand. So you aren't using _any_ of InfoPath's abilities other than a simple way to fill out a form, right? You are just opening the template (XSN) locally, filling it out, then saving it as a data file locally (XML). You are then right-clicking on the file and choosing Send To > Mail Recipients? Yeah, of course that's going to send the attachment. That's just like if you do it with a Word doc or any other file. this is NOT how you use InfoPath, so that's the whole problem.
You should be using all the built-in features of InfoPath to automate all of this. First of all, if you open the template and click File > Publish, you will see that you can actually publish this to a list of email recipients. However, that's primarily so that they can fill it out, which is not what you want, but keep it in mind for later. The other thing you can do is publish it to a network drive. This is what you should do, I believe:
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Drag a plain button to your canvas
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Double-click the button and click Rules, then click Add
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Add an action and choose Submit using a data connection
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You have none, so click Add
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Create a new connection to: Submit data
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Select "As an e-mail message"
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Type in the email address, alias, or display name of the group these always go to and give it a subject (notice these fields can be filled with data from the form if that's what you prefer)
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On the next screen, this is where you choose to Send only the active view of the form and no attachment! This way, they see the form contents without getting an attachment, and they can't modify it.
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Right-click the template and click Design
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Hit Next and Finish
You won't need to publish it anywhere, since no one is using it, but in the future you'd probably want to publish to email or publish to a network location if you don't have SharePoint.