Let's look at two examples of games that IMO do quite well to deter this behavior:
Clash of Clans: In Clash of Clans, new players are given a small amount of starting gold and elixir and a 3-day shield. This shield prevents other players from attacking the new player, but is removed if the new player attacks other players. This gives the new player 3 days to accumulate resources and upgrade buildings enough for a decent beginner-level defense.
- Progress can be measured roughly in two ways:
- Town Hall level. The Town Hall limits what buildings are available to a player as well as what level the buildings can be upgraded to. A player must intentionally upgrade his/her Town Hall in order to progress to bigger and better bases.
- Trophy count. Attacking someone else and winning gains you trophies. Losing causes you to lose trophies, though you won't lose nearly as many trophies as you would have gained by winning a round. Additionally, the player defending either gains or loses some trophies depending on how the battle went.
Now players are loosely separated by their Town Hall level as well as trophy count. The only thing that matches two players up is if the players have a similar trophy count. If someone with a level 2 Town Hall has 1000 trophies, then they will be matched up against other people with ~1000 trophies, regardless if their Town Hall is level 2 or level 10. This creates a soft barrier, as lower level players will run into a point where they cannot gain trophies past a certain point due to higher-level players primarily occupying that trophy bracket.
Additionally there is less benefit for higher level players attacking lower level players. The difference in Town Hall level determines how many resources a particular player can steal. If a level 5 player attacks a level 4 player, then they can only get 90% of the resources available. If they attack a level 3 player, that number drops to 50%. That means it becomes significantly less profitable to continue attacking players with a lower level to you due to the fact that you will end up spending more resources to attack for less resources in return.
Runescape: Yes, yes, we're talking about Runescape here. More specifically I'm going to be talking about oldschool Runescape due to the fact that last I played was somewhere in 2008 a little time after the wildy was removed and player trades sucked. More specifically, I'm going to focus on the wild and its mechanics instead of the non-combat areas.
In Runescape, player-vs-player combat is limited to an area called the Wild. This is an area that starts off at level 1, and progressively gets higher-levelled the deeper (further north) you get. This level determines the difference in combat levels that PvP is allowed. That means in level 1 Wild, a level 90 character could attack another character with a level between 89 and 91. Consequently a level 30 wild would allow much more varied combat scenarios.
This is in part due to the nature of Combat Level. Combat Level is a cumulation of various individual stats on a character (Attack, Strength, Defense, Prayer, Ranged, Magic). Each stat maxes out at level 99, with combat level maxing out at 126. Someone who has level 99 ranged and the rest of the stats are level 1 is going to have a lower combat level than someone who has level 60 in attack/strength/defense, meaning this "Glass cannon" can go toe-to-toe with Mr. NothingSpecial.
People lose gear when dying in the wild. Killing another player will allow you to pick up whatever gear they were wearing. If they haven't attacked anybody else, they still keep 3 items. If they have participated in PvP, then they keep nothing (unless they use a specific prayer). This creates an element of risk. Better gear means higher chance of survival, but a higher risk if you get killed.
Consequently the added risk makes the Wild a good place to put good rewards in. For example, certain creatures are restricted to the wild that yield a high profit.
Depending on the exact nature of your game, either of these two ideas could give you a good sense of scale. Essentially you want it so that higher-level characters gain less by attacking lower-level characters, and gain more by attacking higher-level characters. Adding in some form of controlled risk allows players to progress at their own rate.