The result lifted United one spot to 11th in the standings with 34 points from 28 matches - one point behind the Urban Warriors.
Ajax threatened first in the ninth minute when Tashreeq Morris ran onto a headed pass from Eleazar Rodgers, but his shot lacked the power to trouble Shu-aib Walters.
The Team of Choice had a similar chance in the 18th minute as Heinrich Isaacks was played through by a header from Terrance Mandaza, but Anssi Jaakkola came off his line to deflect his effort wide.
Jaakkola was called into action again in the 26th minute as he reacted quickly to stop a header from Mohammed Awal from creeping into the bottom right corner.
Granwald Scott and Toriq Losper tried their luck from outside the box in the 32nd minute, but both efforts sailed over the crossbar.
United had two late chances in the opening stanza but they also failed to hit the target with Mandaza heading wide in the 35th minute, while Nhlanhla Vilakazi did the same in the 43rd minute.
The opening goal came in the 49th minute when Jayiya volleyed the ball into the back of the net from inside the box after he was set up by a header from Isaacks.
Walters made a good stop on the 55th minute to keep his side in the lead, diving to his left to stop a shot from Scott.
Steve Komphela's men doubled their lead in the 64th minute as Morton converted calmly from the penalty spot after Nazeer Allie was adjudged to have fouled Vilakazi inside the box.
Ajax tried to get back into the game but Walters made a great save to stop a long-range effort from substitute Koen van de Laak.
Both goalkeepers came out on top in the 80th minute, with Jaakkola using his feet to deny Mandaza, while Walters produced an acrobatic save to tip Rodgers’s header over the crossbar.