Have them killed. From the N.Y. Times via AP:
SAN DIEGO ã A former Internal Revenue Service agent whose tax
> preparation business catered to a wealthy clientele is accused
> of ordering at least two former customers killed as they
> prepared to testify against him on fraud charges.
>
> Federal prosecutors say the targets were key witnesses against
> Steven Martinez, 50, who was charged last year with stealing $11
> million by preparing bogus tax returns for his customers.
>
> Martinez's limousine driver ã Norman Russell Thellmann, 64 ã was
> charged Monday with conspiracy to tamper with witnesses.
> Prosecutors allege he was ordered to deliver money to a hit man
> who was promised $100,000 for the two killings.
>
> Martinez did not enter a plea during his initial court
> appearance Monday on a charge of witness tampering. A federal
> magistrate judge ordered him held without bail....
>
> "He had a very dedicated loyal clientele," [defense attorney
> David] Demergian said. "He was very successful."...
formatting link
As a former tax preparer, I'd have thought this would discourage repeat business.