Acrobat treats link underlines as artifacts, so do this to get rid of them:
1. Click on Tools (on the upper right-hand corner of the screen).
2. Expand the Content menu, and select "Edit Object".
3. Right-click anywhere on your document, and select "Find..."
4. The "Find Element" pop-up will display.
5. Ensure "Artifacts" in the "Find:" field, and "Search Page" radio buttons are selected, then press the "Find" button.
6. Find your underlined link in the page. It should be selected (light blue glow surrounding it). Click on the underline and drag it away from your hyperlink.
7. Right-click on it, and select "Delete" from the list of options. Your hyperlink should no longer be underlined after that.
Note that with these instructions, you can also modify the underline to be shorter, longer, etc. I know this answer is late, but I was looking for the answer myself online and didn't find anything. It wasn't until I started playing with Acrobat Pro and realized that the underlines were an artifact that I was able to think of this problem from a new perspective. Perhaps now, when someone runs into the same problem, at least now there'll be a solution. :)