Right-click and select Remove Hyperlink
. This will work on more than one cell at a time, if selected.
If you want prevent Excel from automatically inserting any hyperlinks, you can turn it off in in the AutoFormat As You Type options. In Excel 2016, these are found under File > Options > Proofing > AutoCorrect Options. Select the AutoFormat As You Type tab, then uncheck the box "Internet and network paths with hyperlinks".