不⋯⋯了
bù …… le
Expressing "not anymore" with "le"
Wrap a negative in 不…了 and you get "not anymore" — a change from doing to not doing.
不 / 没(有) + Verb Phrase + 了
How it works
我不想吃了 is "I don't want to eat anymore" — I did want to, and now I don't. Without the 了 it's simply "I don't want to eat".
This is the change-of-state 了 doing its usual job, applied to a negative. 我不去了 means the plan is off.
With 没, the same frame means something stopped being the case: 他没钱了 — he's out of money.
Examples
wǒ bù xiǎng chī le
我不想吃了。
I don't want to eat anymore.
wǒ bú qù le
我不去了。
I'm not going after all.
tā méi qián le
他没钱了。
He's out of money.