Ok, this sounds ridiculously basic, but I don't know how to do it and can't find a guide that just focuses on this one issue.

I'm running a Medieval game and want to limit the clothing to patterns that are suitable. All I want to do is remove certain pre-set patterns on some clothing, which don't look right - Sixties psychedelics etc.

I know I can do this in CAS by hand, but I'd like to control what patterns are applied in random clothing too.

I don't really want to create new objects because I am completely happy with everything else about them.

Is there a simple-ish way to do this?

There is a way that involves using TSRW, S3OC and S3PE.

The steps, in short, would be:

1. clone the item whose presets you want to change, with S3OC (making sure not to rename/renumber it). Leave this file aside for now;
2. open TSRW and start a new project with this clothing item (don't import the cloned one);
3. edit the presets as you see fit. You can add or remove presets to your liking;
4. export your package from TSRW;
5. open the TSRW package in S3PE, open the CASP resource, and export the XMLs for each preset;
6. repeat step 5 for the package you cloned in step 1. Name your XMLs something different so you don't confuse them;
7. open both XMLs for the first preset, copy the pattern data from the TSRW package's XML to the cloned package's XML;
8. import the cloned package's XML back.

Why all this running around? Because TSRW changes your resources' instance IDs, which makes the game unable to recognize them as default replacements.
However, you need TSRW to make new presets (it's the easiest to use program I've worked with in terms of editing presets).

I can write a more detailed guide if you're interested in taking this route. It's not difficult but it is annoying.