Learn Finnish: Sopimussakko on valtava, joten emme riko pykälää. - The contractual penalty is huge, so we will not break the clause.

You could frame it as a condition rather than a consequence: Kunhan emme riko pykälää, vältymme valtavalta sopimussakolta (As long as we don't break the clause, we will avoid a huge contractual penalty). In this sentence, joten (so) introduces a result, whereas kunhan introduces a requirement or condition.

Why is the object pykälää in the partitive case?

Because the verb is negative (emme riko). In Finnish, whenever the action of a transitive verb is negative (meaning the action does not happen), the object of that action must always take the partitive case.

Could we use siksi instead of joten to mean "so" or "therefore" here?

Not using this exact punctuation. Joten is a conjunction that links two clauses together in the same sentence. Siksi is an adverb, so it typically starts a brand new main clause: Sopimussakko on valtava. Siksi emme riko pykälää.

Why does the verb rikkoa (to break) drop a 'k' in emme riko?

This is due to consonant gradation. The infinitive rikkoa has the strong grade (kk). The negative present tense uses the weak stem (the same stem used for the first-person singular "I do"), which causes the double kk to weaken to a single k.