Given Janemba is a being made of souls and spiritual dark energy, who can control said energy, he should be able to kill DS here. It makes sense to given characters much weaker like Kami, the past Kamis, King Piccolo and his clan, Garlick Jr. etc. can all attack souls in various ways, and basic ki blasts from Kid Buu could also kill the souls in other world, so attacking souls is nothing new in DB.
Janemba is faster by far, (Way over 7,000 times ftl for DS vs Way over 4 quadrillion times ftl for Janemba), and speed not equal makes him blitz DS, and he is higher on the galaxy level scale, nearly reaching to multi-galaxy level (although imo he is actually well into 3-B, but whatever).
Given he has soul manipulation, is faster and stronger and DB characters are known for being able to kill guys with Regenerationn like Buu and Cell (Goku and Vegito both said they could kill Buu), and dimension sword outright ignoring durability, I am actually going to say Janemba has the tools to win consistently with a blitz before DS can use his offensive hax, the power to bust open all the dispel bounds at once like Uriel did, and the soul manipulation to actually put DS down for good.
DS has some hax that could potentially beat Janemba like the Judas Priest skill, but Janemba resists soul manip and has more durability so even that's questionable. Also DS will not likely ever hit him even if Janemba couldn't put him down right away, so at best even if you say DS can take anything he dishes out, it will be Janemba wailing on him till DS tires from using the Judas Pain and taking the damage, reverts back to base and gets slaughtered or sealed like Yemma imo.