Especially for sources in footers I often want to avoid re-typing a complicated title of a document that I found on the web. Copy-paste of the text also copies some of the text formating. Solution: copy the text, paste it into "Note pad" (the standard text editing utility that comes with Windows), select the text again and paste it into a PPT text box. I would be interested if other people have faster solutions for this.
UPDATE: Remy got the solution in the comments: copy any text, then select Edit and "Paste Special" and select unformatted text. Thank you!