You cannot claim a username taken by someone else unless they are impersonating you.

If you find that the username that you wanted has been already taken, adding numbers, underscores or abbreviations can help you come up with a great available username that works for your trademark.

Using a company or business name, logo, or other trademark-protected materials in a manner that may mislead or confuse others with regard to its brand or business affiliation may be considered a trademark policy violation. If you are a legitimate trademark holder, and someone is misleading users, please get in touch with SlideShare support  and we can look into this on a case by case basis.