The sentence construction “(noun) (verb phrase) by (noun)” is known as passive voice or passive construction, because the true subject is relegated to the end of the sentence and is thus acted on, rather than acting, which often weakens the statement.
The solution is simple: Give the focal point of the sentence its due—”(noun) (verb) (noun),” and demote the false subject to the back of the line. Note that not every passive construction is evil — sometimes what seems to be the false subject is worthy of prominence—but a preponderance of passive constructions leads to a wearying read.
1. “There is a considerable range of expertise demonstrated by the spam senders.”
The actors in this little drama are the spam spenders—or, to be more active, the spam senders are the actors in this little drama. Direct them center stage, and send the weak “there is” opening packing to the provinces: “The spam senders demonstrate a considerable range of expertise.”
2. “It was determined by the committee that the report was inconclusive.”