Jozi finished second with 29 points, while Paarl ended in third spot on 22 points and now bow out of the competition.
Morkel slammed four fours and five maximums in his 57 not out off 28 balls.
The Paarl side, who got to their target in 15.2 overs, will now go on to face the Jozi Stars in the Qualifier on Friday.
Rassie van der Dussen and Dan Christian produced a 118-run partnership which powered the Stars to 239/3.
It was only a second win of the campaign for them. Though, they remained rooted at the end of the points table.
In reply, a 120-run stand between Du Plessis, who made 61 off 34 balls, took them too far ahead for the Stars
Cloete struck 65 from 59 balls which laid the foundation for the Spartans’ 165/4 in their total of 20 overs.
The Proteas opener smashed three sixes and 12 fours in his 47-ball cameo as the hosts chased down a 145-victory target with 4.3 overs to spare.
Two consecutive sixes in the final over from Jozi Stars’ Nono Pongolo saw his side to a one wicket win over the Durban Heat
Shadab Khan ruled out of England series with virus, Yasir Shah named replacement
Shinwari congratulates World Cup squad
Key for Yasir is to challenge both edges of the bat, says Arthur
Mohammad Hasnain journey to World Cup
Ali Zafar comes out in support of Mohammad Amir
Smith, Warner named in Australia squad for World Cup
WC squad meets PM Khan to seek inspiration
Bowling conditions in England would make it tough for Bangladesh, says Walsh
Sarfaraz to bat at No.5 in World Cup
PSL 2019 - PSL Schedule with all PSL 4 matches and schedule for all franchises