The detail that conservatives tend to forget is that when one party violates a contract, the other party is no longer bound by all of its terms. If you sign a contract to buy a car, and the dealer refuses to turn it over you, you aren’t “sinking to their level” by refusing to hand over your money. If you contract an employee who never shows up for work, you aren’t “repaying evil for evil” by withholding his wages. The same is true when dealing with people who are deliberately uncivil to civil people — it fundamentally changes what the rest of society owes them.
In other words, the aggressor makes the rules. We don’t ‘stoop to their level’ by fighting aggression by their rules.