There is no real way with plugins to make a player fly with a rank, but with essentialsX you can make it so they can execute a command to fly! Just follow what coolprogrammer said to do.
Heres what you can do to prevent.
Whitelist your server! you must be op
You should also ban-ip him using /ban-ip Tramskovi
You also need to be operator
This makes it so he cant join on his main and alts.
I dont really recommend buying skripts from the minehut marketplace. alot can easily be skripted, and most can even be found in 1 google search, for free