This is an official BML 

An *official* BML? Color me suitably impressed. :-)

I give. What's a BML?

why she wouldn't want to rely on other people. But enforcing her
contracts by beating the snot out of the offending party is something I
really have trouble seeing her doing.

I don't. Nabiki has a mean streak. She does work out too. 

I can see her getting personal with someone who is trying to cheat her.

I can see her taking care of business herself because:
A- Don't have to split the profit of pay someone to do enforcement
B- Keeps the mystery in Nabiki's methods. Who's gonna blab?

Nabiki is fairly fearless, this would explain why. 
She is self-sufficient against common foes.

Don't you think she could come up with something a little less crude?
"Oh, don't worry about it, Tezuri. If you don't have the money, you
don't have the money. I'll just have to recoup my losses by selling some
of the photos I took of you last Thursday behind the.... What's that?
You found the money? Oh, good!"

This is also viable... but if she doesn't have something on the guy, that
may not work.

I know about a fic where she bluffs a guy with pictures (that were blank)
and he fell for it... but if this guy is sure about himself, that bluff
will fail and she'll lose credibility.

This guy seemed quite sure of himself, I suspect that bluff would've
failed. Nabiki would probably have calculated such a risk before acting.

Anyway, those are my thoughts.

  ___________
 / Here lies:\  Time files... until the clock hits the floor.
|   Andrew    | 
|             | 
|   I think   |       Andrew Petalik {ICQ ID: 160869}
| he's hungry | Jadzia Dax    : Don't worry, I have a light touch. 
|             | Julien Bashir : Not according to Worf...