Suppression of a ligature in XeLaTeX

This is possible using Mico's selnolig package. It is on CTAN, and added to TeX Live and MikTeX. selnolig requires the document to be compiled with lualatex, but that wouldn't keep you from using your fonts; indeed, it is pretty similar to xelatex in usage.

(Btw, microtype is available for LuaTeX -- most of its functions -- but it cannot selectively deactivate ligatures. That's why selnolig was created.)

In this MWE, I load selnolig without either of its language options (it can deal with German and English), so it only gets rid of ligatures where I tell it to with \nolig rules. Since I don't have the Adobe Caslon Pro OTF fonts, I used Linux Libertine, which has the ⟨st⟩ and ⟨ct⟩ ligatures in its Historical set.

\documentclass{article}
\usepackage{fontspec}
    \defaultfontfeatures{Ligatures={Historical}}
    \setmainfont{Linux Libertine O}
\usepackage{selnolig}
    \nolig{st}{s|t}
\begin{document}
best pact 
\end{document}

Just as a comparison, here's the output without selnolig:

selnolig's main purpose is to automatically suppress ligatures across morpheme boundaries in German and English texts. Its documentation explains all about how to use it (and how to use lualatex, if you'd like some pointers there).

See also New package, selnolig, that automates suppression of typographic ligatures on Meta.

@doncherry has already addressed your main question. I'll take a stab at your second set of questions:

Another question I have is, is it possible to change ligature options midway through a document? Or perhaps temporarily change the ligatures for one occasion?

You're not very specific as to which of potentially very many ligature-related options you're looking to change, so my answer has to be quite broad. I think your best bet is to use the \newfontfamily command of the fontspec package to set up a "new" font (which could be the same as that set by \setmainfont...) and some font-related options. For instance, you could have the following setup in your document's preamble:

\usepackage{fontspec}
    \setmainfont[Ligatures={Common,Rare}]{Adobe Caslon Pro}
\newfontfamily\acptemp[Ligatures={NoCommon}]{Adobe Caslon Pro}

You could then issue the commands

\bgroup
\acptemp
...
\egroup

inside the document's body to switch to a setup with all ligatures disabled within the scope of \bgroup ... \egroup -- while keeping the same basic font.