ebgaramond remove Q-"swash" in Xetex

You could add the feature to the default features for the roman family before loading the style:

\documentclass{article}

\usepackage{fontspec}
\defaultfontfeatures+[\rmfamily]{RawFeature={-calt}}
\usepackage[lf]{ebgaramond}
\begin{document}
Quack --
\end{document}

(curiously the feature is by default active only with xelatex but not with lualatex).