Narrative:

Aircraft X was put in holding at jli for sequencing to lax. I gave them holding as published. The published holding is left turns and he made right. I was told to give them one turn. So after make half of the holding pattern I gave aircraft X a right turn direct vista and maintain 28000 feet which they read back. I had him stopped them at 28000 feet because aircraft Y was climbing on the SID out of san to 27000 feet. I then assigned aircraft X 250 knots. They read that back and did say knots and not just 2-5-0. A moment later I saw aircraft X descending. I told them to stop their descent at 25000 feet. They didn't say anything. I stopped aircraft Y at 24000 feet which he was at 23800 feet at the time and said they would go back down. I went back and stopped aircraft X at 25000 feet again. I turned aircraft Y 10 left and expedite his turn. I turned aircraft X to a heading of 160 which I had to say twice. I called traffic to aircraft X. No response. I called traffic to aircraft Y and he said traffic in sight. I went back and called traffic to aircraft X again. After they were pulled apart I gave aircraft X direct vista again and down to 24000 feet. I then gave him the brasher warning and had given him the phone number 3 times. Almost every clearance I gave to aircraft X I had to say twice. [Suggest] maybe restate the altitude when I issued the speed. But I did make sure I heard the word knots and not just 2-5-0. So I don't know. Maybe extra separation when obviously the pilot doesn't want to listen.

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Original NASA ASRS Text

Title: ZLA Controller reported a pilot deviation descending from an assigned altitude; which resulted in being in conflict with climbing traffic.

Narrative: Aircraft X was put in holding at JLI for sequencing to LAX. I gave them holding as published. The published holding is left turns and he made right. I was told to give them one turn. So after make half of the holding pattern I gave Aircraft X a right turn direct VISTA and maintain 28000 feet which they read back. I had him stopped them at 28000 feet because Aircraft Y was climbing on the SID out of SAN to 27000 feet. I then assigned Aircraft X 250 knots. They read that back and did say knots and not just 2-5-0. A moment later I saw Aircraft X descending. I told them to stop their descent at 25000 feet. They didn't say anything. I stopped Aircraft Y at 24000 feet which he was at 23800 feet at the time and said they would go back down. I went back and stopped Aircraft X at 25000 feet again. I turned Aircraft Y 10 left and expedite his turn. I turned Aircraft X to a heading of 160 which I had to say twice. I called traffic to Aircraft X. No response. I called traffic to Aircraft Y and he said traffic in sight. I went back and called traffic to Aircraft X again. After they were pulled apart I gave Aircraft X direct VISTA again and down to 24000 feet. I then gave him the brasher warning and had given him the phone number 3 times. Almost every clearance I gave to Aircraft X I had to say twice. [Suggest] maybe restate the altitude when I issued the speed. But I did make sure I heard the word knots and not just 2-5-0. So I don't know. Maybe extra separation when obviously the pilot doesn't want to listen.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.