KOMA-Script decides in the default setting that an “end dot” will be used if any section numbering macros (\thepart, \thechapter, …) use Roman numbers or any letters.
When you change \thepart to use \arabic, it will switch automaticaly to “no end dot”.
Do you want
- to never have an end dot, or
- to not have an end dot with Roman part numbers?
No end dot in any numbers
You can use the numbers class option with the value noenddot.
See
Code
\documentclass[numbers=noenddot]{scrbook}
\begin{document}
\part{Mytitle}
\chapter{My other title with no end dot}
\end{document}
No end dot in the part number
If you only want the part’s number to not be followed by a dot but all other section numbers followed by one, KOMA-Script does not provide an interface for that.
My proposal:
Use the default setting for numbers (i.e. auto; because \thepart still uses \Roman all other section numbers will get the dot either way) and just overwrite \partformat:
\renewcommand*{\partformat}{\partname~\thepart}
%\newcommand*{\partformat}{\partname~\thepart\autodot}% <- original defintion in scrbook.cls
This will remove \autodot from the definition.
Code
\documentclass{scrbook}
\renewcommand*{\partformat}{\partname~\thepart}
\begin{document}
\part{Mytitle}
\chapter{My other title with an end dot}
\end{document}
Output (both)

\partformatwill not be redefined by KOMA-Script anywhere, so it should be safe. – Qrrbrbirlbel Mar 13 '13 at 16:34