Honestly at my tier in arena I hate seeing Ninja more than UDK. That said I do make sure when he is on the other team to run AOE champions or champions with multiple attacks. 

I just leaving him till the end and then burn him down. 

Look for and develop your first Block Debuffs champ and things will go much faster. Two obvious ways of dealing with UDK at the lower levels are Rhonda and most bomb champs. Sounds wierd but you can bet as soon as that UDK has access to Stoneskin its going to add to the headache. Bombs ignore SS and when the skin wears off UDK is all but dead.

If you have champs like Pythion, etc. Those also take much of the punch away real quick but we do see UDK all the way up through Gold and Platinum on a regular basis. He becomes a basement dweller most of the time so there is hope here, just requires a little patience. 

Good luck and keep asking questions!

quitting? maybe

but I think the motive is to get you angry so you can think what can I buy to fix this?

That too. But I tend to think it convinces more folks to quit the game, the prices on some stuff is insane.

I mean, look at some of the items in the shop. The Veteran Progress pack gives you 3 Voids, 1500 gems, 20 Energy, 10 Ancients, and one Sacred as you level from 30 to 55, all for $99.99 

I mean, seriously? You can buy a Nintendo Switch for that.

I already quit the game a few times. only to return in about 6 months.

I did lose out on a few free chanps, like ninja.

I disagree. If you don't want him to use his passive in PvP then either don't bring in champions that passivly heal themselves and others or use someone to block his passive like Ronda.