In themaPoster for prefixes you would need to make sure that in "CONFIG" -> "Prefixes", the prefix tag (not title) is the genre name (the same as it appear from IMDB).
Then make sure that prefixes are synced in both programs (they are synced on start-up always or via "Sync" menu (at the top) -> "Program name" -> "Sync prefixes").
In themaCreator "Main" template you can add this line:
Code:
<prefix>{#imdbGenres#}</prefix>
And with replacement you would need to remove additional genres to leave only one.
Search for:
Code:
<prefix>(.+?),.+</prefix>
Replace with:
Code:
<prefix>$1</prefix>
Enable regex search.