My impression from brief research is that in 1981 then-Senator Proxmire got an amendment made into law that forbade NASA from spending any money on SETI. However, Carl Sagan got him to change his mind a year or so later.
Then, in 1992, NASA started a formal SETI program, formally called the High Resolution Microwave Survey. However, a year later, Congress eliminated funding for it.
The present situation appears to be that NASA is not forbidden from researching SETI, but Congress is not funding any programs to do so.
There's an article about NASA and SETI
here (NOTE: PDF file), from which I got most of this information.