Formal:
* Unable - "I am unable to attend the meeting."
* Incapable - "She is incapable of lifting that weight."
* Not able - "He is not able to complete the task."
* Cannot - "The system cannot process your request."
Informal:
* Can't - (This is still the most common and informal option)
* Don't - "I don't have the time."
* Won't - "I won't be able to make it."
* Not going to - "I'm not going to be able to go out tonight."
Specific situations:
* Prohibited: "You are prohibited from entering this area."
* Impossible: "It is impossible for me to be in two places at once."
* Refuse: "I refuse to answer your question."
* Unwilling: "I am unwilling to help."
To give you the most accurate suggestion, please provide the context or sentence where you want to use the word "can't."