Complete the circle with a higher-level slot

This is the obvious implication of the linked question: the spell level of the permanent teleportation circle is the spell level of the final casting that makes the circle permanent. Higher-level spells are more difficult to dispel.

Assuming a large organization is setting up the circle, the highest-ranking magic user in that organization may have access to spell slots higher than 5th level. If so, they should use the highest level spell slot they can to complete the circle. For flavor, this practice might be enshrined in a "christening ceremony" of sorts, where the leader of the organization (who also happens to be the highest-level caster) is called upon to complete the work of setting up the circle.

Limit physical access to the circle

However, that doesn't stop a dedicated adversary, who could potentially make many attempts to dispel the circle, eventually succeeding by chance even if it was cast by at 9th-level. So, any permanent teleportation circle will be guarded. The DMG makes this point as well:

However, since every teleportation circle is a possible means of entry into a city, they’re guarded by military and magical protection.

The DMG gives a different reason for guarding the circle: to protect the city from attackers using the circle to bypass the city walls. However, it is also logical that the organization in control of the circle will want to protect their investment from being ruined.

To that end, it makes sense that they will not let just anyone see the circle. Dispel magic requires unobstructed line of effect to work, and preventing adversaries from ever having line of effect to the teleportation circle is the most effective countermeasure against tampering. The circle will be placed in somewhere that is not visible from publicly accessible areas of the building. Anyone trying to tamper with it will have to sneak past the guards and other defenses (including magical alarms and wards) in order to get line of effect. The circle may even be in its own dedicated room, protected by a locked and reinforced door (which will likely be further reinforced by an arcane lock spell).

Dispelling the circle is not the worst an enemy can do to it

Lastly, it's worth noting that if an enemy knows about the teleportation circle, dispelling it may not be the best course of action. As the DMG notes, teleportation circles are potential entrances. To me, it seems more likely that an enemy would attempt to gain knowledge of the circle's sigil sequence and attempt to leverage that knowledge, rather than just attempting to ruin the circle. Depending on the nature of the enemy, they could sell the sigil sequence to the highest bidder, hold the organization ransom, or use it themselves to help their evil organization infiltrate the city. But any possible value they could derive from knowing the sigil sequence is destroyed if the circle is dispelled.