Formal:
* Non, je suis occupé(e). (This is the most literal translation and is appropriate for most situations.)
* Je suis désolé(e), mais je suis occupé(e). (This is a more polite way to say no, using "désolé(e)" to express regret.)
* Je suis pris(e) par mes obligations. (This means "I am taken by my obligations" and is a more formal way to say "busy".)
Informal:
* Non, je suis débordé(e). (This means "I'm overwhelmed" and is a more informal way to say "busy".)
* Je suis trop occupé(e). (This is a simpler way to say "I'm too busy".)
* J'ai pas le temps. (This literally means "I don't have the time" and is very informal.)
You can also use a combination of these phrases to create more nuanced responses, such as:
* Non, je suis vraiment occupé(e) en ce moment. (No, I'm really busy right now.)
* Je suis désolé(e), je suis pris(e) par le travail. (I'm sorry, I'm tied up with work.)
* Je suis occupé(e) pour le moment, mais peut-être plus tard ? (I'm busy at the moment, but maybe later?)
Remember to adjust the level of formality and politeness based on who you are speaking to and the situation.