I suppose if storage space is a problem then you could free up some space by removing the search index records. Or you could buy more storage from your host which is usually very cheap.
In my testing I found that XenForo maintains the search index records even if the search engine is disabled. I don't see an option disable this.
You can always manually empty the search index from time to time with this query:
Code:
TRUNCATE TABLE xf_search_index;
Then you can rebuild the search index if you ever decide to use the search feature:
Admin CP -> Tools -> Rebuild Caches -> Rebuild Search Index