What are the reasons this happens?
Does it mean stay away?

Actually, not totally.

I don't really have a definitive answer but from an experience I can share, I will say that zero-reviews or a de-listed review does not equal RIPOFF.

One example is of a nude/sensual bodyrub agency in Tysons.  They aren't a FS provider but they give erotic rubs and happy endings.  They had girls reviewed quite frequently years ago.  All of a sudden the reviews not only stopped they disappeared.  I happened to still visit them after this observation and found that they were the same company, with the same quality I've enjoyed over the years.  My only guess was that the Tysons-based company somehow requested TER to remove the ads.

I guess it would be a TER admin that could give you a clearer answer.

Sometimes the provider requests that her profile get removed.

Not necessarily. Thay may just be UTR or had some drama with TER and gotten themselves banned for rule violation.
They may just want more privacy.

There are two statuses here.  Let's look at them one at a time:

Delisted -

The easiest to explain.  The Provider has asked TER to remove all of her reviews.  This can be for any reason (examples are quitting the business, having been outed to family, etc.) - EXCEPT for ROB/Ripoff reviews, TER will not delist that provider.

Banned -

One of the most common reasons for banning is getting caught for self-reviews.  This is grounds for immediate banning.  Others are rules breakages like outing hobbiests or other providers, spamming people using the PM system, and various and other rules violations.  A little known reason is for refusing to allow reviews of herself.  TER is about reviews.  If the provider will not allow reviews, then she is not welcome here.  another way is to be belligerent with staff about having "fake" or poor reviews removed, or for other reasons.

Now, what does this mean?  Well, the bottom line is that providers in either category are not welcome to post (ads or otherwise), they may not be reviewed, and we're not allowed to discuss them on the boards.  

The issue about not discussing them usually causes some controversy, but the reason is simple - why give them "press" if they have removed themselves from TER, or have shown disrespect by rules breakages or whatever.  It is as simple as that.

The final question that always arises is - "How do we know they are banned or delisted?".  Not even the mods have a full list.  We know from experience.  One telltale sign is to click on the providers profile (when they show up in a reviewers list), and get the "Provider Profile no longer available" message.