Possible fix: IIRC Niven's Ringworld sported very powerful defenses to thwart asteroids. We know, canonically, that these were common for even planet-bound Ancient installations, both for defense and traffic control. We also know, from canon, that it is possible for non-Ancients to gain control of them.
I'm citing
CT Adventure 3: Twilight's Peak here obviously. For those who don't have it handy: a physical (?) globe of Fulacin is augmented with holographic (?) display of nearby ships down to a resolution of 60m (scale: 0.1mm). Using control consoles nearby, the display is augmented (visually? telepathically?) to 100 times the previous scale. Using other controls, a representative (?) sphere can be made to appear, and moved within 1m of the globe. If this overlap ships and is released, the ship disappears leaving no trace or debris. (I misremembered the bit about traffic control, possibly thinking of the gateway controls in T4's
Long Way Home.)
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Challenge #39 has some details about the Stalkers. They had sublight ships and acquired Jump drive from the Solomani (in theory); it's therefore probable they've been in control of Leenitakot for thousands of years, before the major powers (Vilani, Solomani, K'kree, Hivers) appeared in Hinterworlds.
The Stalkers are not warm to intrusions into their realm. This has led to frustration in the xenoarchaeological community as within the borders of the Outcasts of the Whispering Sky is the Leenitakot system (1432). Both Imperial and Solomani scientific communities are constantly negotiating for exploration rights, but to date only four have been granted.
The Stalker policy on unauthorized intrusion is simple: Destroy the intruders. Regardless, many free traders have risked their lives penetrating deep within the Stalker worlds for some unique spices and plant life which can be found nowhere else.
Taking this as canon, it would seem the Stalkers are not under threat from fleets vying for control of the ring.
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If I were writing the adventure:
The Stalkers exploring the ringworld noticed G-band interference which led them to a control room. The room has a scale representation of the ringworld and the system out to 2 radii from the central star. Anything in this moving under power or large enough to threaten the ring appears as a holographic display. Similar to the control room in the Ancient complex at Fulacin (although with an entirely different set of controls), similar "erasure" bubbles can be deployed to wipe threats from existence. Ships can also be relocated by "control" bubbles, which allow a pocket of space to be repositioned instantaneously (no light-time lag, no acceleration) anywhere within 2 ringworld radii from the central star.
Since the Stalkers discovered this, the control room (one of six...) has been continuously staffed and operated, preventing any unauthorized visitors anywhere near the ringworld. A position as one of the 36 monitors is the highest honor a Stalker can have; it is the apex of the cultural paranoia/curiosity about the stars.
Each of the major powers has attempted to explore or take control of the Ancient artifact several times since learning of its existence, with efforts ranging from single scout ships to fleets. In every case, the intruders have been destroyed, with no warnings given and no trace ever discovered. The Stalkers do not see this as punitive, simply as a cleansing. Expedition rights have been granted in the past and there is every indication that they may be again. While unwelcome intruders are dealt with quickly and efficiently everywhere in the polity, the Stalkers are not completely insular.
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The Hivers, of course, are totally playing "us non-verbal communicators need to stick together, yo" angle and working long-term to influence the sect that staffs the ringworld control.
