There are way to constrain the most powerful spells for the dark and/or evil witches. Individual methods will solve individual issues and play on a different aspect of the world.
The Most Powerful Magicks Require a Sacrifice
If a spell requires the sacrifice of a sentient, then good witches are either going to be reluctant to do it or the assumption is that it will be their own sacrifice that powers these spells. This plays on the differences in morality beween the two variants of witches and allows for interesting moral situations.
It may not be necessary the sacrifice of a sentient either -- it just has to be a price that the average good witch is unwilling to pay in the normal course of events. Those that know of these magics will be burdened with the knowledge of them and their costs.
Should these sacrifices leave obvious evidence of their use, then the law-enforcement angle can come into play. These spells without permission from the local leadership is an immediate harsh sentence (likely death or immediate banishment). The trick is to come down hard and fast on those that utilize these sacrifices.
Just Because You Can Does Not Mean You Should
Others have touched on this, but the idea that the runes that bond magics together are the thing to limit. In general, spells do what they are designed to do. Once you deviate from that, then you get into unknown territory. Overall, this deals with the bonding magic itself and the mechanics that this runs off of
The core premise here is that these runes are designed to bind the magics of two beings together. The moment a third is added to the link, complications may ensue.
As other answers have stated, concentration increases when more are added. This makes adding too many people to the link impractical past a world-defined point.
When you pull mana from the link, it pulls from all people simultaneously. The pull stops for all parties once 40% is taken from any one single person as per the safety measures. So if your third person is weak magically, it might end up that the witch actually gains less from two people than they would from one.
The safety measures built into the runes to prevent death by draining actually prevent magic being drained from more than one person at a time for any one spell. Whether this takes the form of not being able to place the runes on a second donor, or not being able to open a bond to a second person while the first is open is up to the magics. By contract, the hallmark of the dark withces are these runes without the safety measures in place.
There are unknown complications when a witch applies the runes to two living beings. Unknown in the sense of they differ every time because of the three magical forces involved. They could theoretically range from actually beneficial to the death of all three participants. Because the side effects are not codified, this is considered a last-ditch effort for good witches
The Long Arm of the Law
If there is enough konwledge of amgic, then the ruling factions will have regulations as per its use. If the magic is powerful enough, then the punishments for using either banned magics or banned methods to using the magic will fit the crime. This plays into the morality of the witches but by creating consequences that loom over them should they step out of line.
It might mean that the bonding runes have to be a specific sequence to be legal, or that certain spells are banned outright. Regardless of what measures are taken, it needs to be demonstrated that the law enforcement has to be able to deal with the perpetrator.
Note that law enforcement does not necessarily have to be soldiers or police. It could be the witches themselves that govern their own actions.
An in-world response might be that witces in general do not consider bonding to multiple people. It might be a societal taboo, or one that is ingranied to them when they learn this spell. A good witch might actually know of this and consciously decides against it to prevent them being branded a dark witch. Regardless of the whys behind it, the key point is that there is nothing that explicitly stops a good witch magically speaking, but something that personally stops them.
In contrast, a dark witch not only knows of this but does this willingly. They accept the consequences of their action and exploit this for their own personal gain and profit.