This is effectively the same general problem of how con you control how it's used?  Ultimately, you can't.  Whether it's used for drugs, gambling, repayments to loan sharks, or any other un-'planned' uses, degens will find a way.  The advocate position - as unfalsifiable as it may be - is simply that on balance, the positive results provided would outweigh the negatives. 

 This is why CBDC and UBI are such a perfect fit.

UBI is the carrot for CBDC, and CBDC is the control on how UBI is used. 

 Even then, CBDC control is only effective until value is transferred to the underground market.  e.g. receive CBDC, use CBDC to buy approved food, clothing, etc; then, trade approved purchases for underground market currency in order to spend on whatever.

In short, Degens will find a way. 

 this is actually somewhat reassuring, but isn't CBDC use totally transparent to those controlling it? 

 Yes.  It almost certainly would be designed to be totally transparent to those controlling it.