This has been asked before: Why doesn't Visual Studio code formatting work properly for Razor markup?
But that question is a couple years old. And Razor formatti
What did the trick for me was pressing CTRL+Z after pasting, then the formatting is removed.
In my case R# ended up being the culprit, can be disabled here:
ReSharper > Options > Code Editing > Razor > Editor & Formatting
https://www.jetbrains.com/help/resharper/2016.1/Reference__Options__Languages__Razor__Editor.html
Under Tools -> Options -> Text Editor -> HTML -> Advanced
there is a key value item Paste which says "Format on paste" and has a boolean value next to it. Changing this to false has disabled formatting on paste for me in razor syntax.
I am using VS Professional 2013, Version 12.0.30110.00 Update 1