There are a couple of options to consider.

 1.  Message formatting > Always paste plain text in preference to styled text.

This setting isn't practical if you sometimes copy/paste the formatting but if you always send unstyled text then this is the way to go.

2.  Use one of the right click paste options to paste unformatted.

I tent to use Arial as my default font and that is what i have set In the Message->fonts setting.  However when I copy and past text from one email to another  it is pasted at Times New Roman which us very annoying.  Is there an easy way to make pegasus use ZArial for pasted text?  My guess is that this is actually a Windows setting that is hidden away somewhere but I thought it was worth asking.

See article: https://www.laptopmag.com/articles/change-default-font-windows-10

 It involves updating the Windows registry, but is not complex

Martin