You can use the "Paste as plain text" (Google Chrome), or paste into notepad, and then copy from notepad.
But easiest of all is to use "PureText"
Download PureText from here: http://stevemiller.net/puretext/
Unzip and save Puretext.exe anywhere you wish (Program Files, My Documents, some utilities folder, etc.) and then just double-click on it to run.
After running PureText.exe, you will see a "PT" icon appear near the clock on your task bar. You can right-click on the icon to display a menu with more options.
If you use a Mac
Use the feature "Format/ Make Plain Text" that does the same job as "Pure Text" (which is windows only)