Can Donald J Trump legally ban TikTok just because he wants to?

No. "because I want to" does not have a legal value, even if the 'I' happens to be the president.

The fat moron only wants to get petty revenge since people apparently used TikTok to coordinate the orders of all those fake tickets for his Tulsa rally.

The most intense app bans happen at the network level, blocking any communication between the targeted servers and users in the country. That’s the approach taken by China’s Great Firewall, and it’s how India enforces its recently implemented TikTok ban. (Australia, which is considering a similar ban, would likely take the same approach.) But American law doesn’t have any precedent for blocking software in that way, so it seems unlikely that the White House would be able to follow through on that kind of heavy-handed network censorship. So, as Trump always says, 'we'll see what happens...' 

If China is really spying on Americans through Tik Tok my guess is it is his responsibility to close it down in the US based on national security.

I wouldn't think that's legal, only way is like the feds cease the site but tik tok isn't one of those illegal types of sites or really app

No, he needs a lot of help if he wants do that

he is scared of communism becaus Nazis fought against them. TikTok is a truly nationalized service