When pasting a paragraph of text into an Excel cell, often times there are “enter” characters that were pasted as well. To eliminate them, use the following formula in another cell:
=SUBSTITUTE(A1,CHAR(10),” “)
The formula replaces the “enter” character in cell A1 with an empty space. The enter character is represented by CHAR(10) in Excel. Now, you can do a “Paste Special” and paste the value over the original cell.
The process is much simpler in Microsoft Word. You would use the “Find and Replace” dialog box, and use either ^P or ^l – depending whether you are replacing an alt+enter or a regular enter.
Source: Removing Line Returns (Alt+Enter) in Excel Cells [Accounting Web]