First of all...I'm not sure why you would think they couldn't? You apparently understand there is a process called Levy that can be done against your account. Whom do you think that process is expected to be used by...and to do what? Maybe collect a debt? And they are an agency doing what...collecting debts.
Now, they certainly would rather have you pay, without having to go through the time, expense and trouble to levy an account.
To levy your account there may be certain process they would have to do first, and be able to prove your debt, etc. But, the Law, the courts, their kids, their school, their grandparents and their doctor, even their Church, etc., etc., everyone they need or want to pay really wants them to be able to collect what they are owed.
But, first off: Did you bother reading whatever agreement they are collecting on that you didn't bother following? (Understand, paying is following...not paying isn't following. Your the one that did wrong). if a right is needed, see if you gave them the right to do so in that agreement....what do you think....the guys writing the contract blank smart enough to put it in, even if it isn't always needed...along with some general language to make sure they can collect?
If you have the money to pay on the debt in the account...why don't you do so?