A key prosecution witness refused yesterday to testify against Dimitrios (Jimmy) Prassos, accused of conspiring to arrange the robbery and slaying of his father, a well-known Ellicott City restaurant owner, sending prosecutors scurrying to find witnesses to link Prassos to the crime.

Seven times during a pretrial hearing yesterday, Lisa Williams, 20, of Columbia, who police said had played a major role in implicating Prassos in the crimes, refused to answer questions, invoking the Fifth Amendment protection against self-incrimination.

In response, Prassos' trial, originally set to start next Wednesday, was rescheduled to Feb. 8. Assistant State's Attorney Richard P. O'Connor said he needed more time to interview two new witnesses who prosecutors say can provide substantial evidence against Prassos and codefendant William H. Ray Jr.

Because of Williams' refusal to testify, O'Connor said prosecutors have revoked a grant of immunity agreement with her.

According to police, Prassos, 23, conspired with Ray, 30, to rob the Pirate's Cove restaurant on Rte. 40 southwest of Baltimore owned by George Prassos, 59, the younger Prassos' father. Ray's trial is tentatively set to start April 4.

Yesterday's hearing was held on several defense motions to suppress statements by Williams, who according to police said she had seen Prassos and Ray, the alleged gunman, together several times in a Howard Johnson's restaurant in Towson where she was a waitress.

In the last few weeks, however, Williams has indicated to police and prosecutors that she wanted to retract her statements implicating the two men, O'Connor said.

Prassos' attorney, Joseph Fleischmann II, told Circuit Court Judge J. Thomas Nissel that Williams' photo identification of Prassos should not be allowed because the evidence was improperly obtained.

Fleischmann said police used "suggestive" tactics to elicit Williams' identification of Prassos on Aug. 24. Two days later, Williams again identified Prassos at the police station through a one-way mirror, without the presence of an attorney for Prassos, Fleischmann said.

Nissel said he would rule today or early next week on the motions to suppress Williams' statements.