Refuse to develop

If the code base is so bad that it's close to FUBAR (a variation on the military slang that stands for 'Fucked Up Beyond Any Repair'), then a more drastic approach may be to refuse further development until refactoring is done. This may seem extreme, given that the people you're disobeying are paying your salary. While this is an easy way to forgo responsibility, it's not what a professional developer should do.

To paraphrase an analogy from The Clean Code by Robert C. Martin: Let's suppose you are a doctor and a patient asks you to perform open heart surgery on him/her in order to relieve a sore throat, what would you do? Of course, you'd refuse! Patients do not know what are best for them, that's why they must rely on your professional opinion.

Likewise, most business owners do not know what is best for them technically, which is why they hired you to make the best possible technical decisions for their business. They pay you not simply to code; they pay you because they want you to bring value to the business. As a professional, you should think about whether your actions are beneficial or detrimental to the business, in both the short and long term.

Business owners also need to trust the advice of their developers. If they do not respect their professional opinion, they shouldn't hire them in the first place.