The most reliable way to halt a bad psychedelic trip starts with non-drug de-escalation: move to a quiet, safe space, reduce light and noise, have a calm, trusted sitter, focus on slow breathing, hydration, and reassuring, simple cues about time and safety. Intervene more assertively only if there is ongoing panic, loss of contact with reality, or risk of harm.
Not every intense moment needs to be stopped. Many difficult phases pass if the person feels safe and supported, and these moments can carry meaning once arousal settles.
If the situation keeps escalating, seek professional help. In clinical settings, some protocols use agents like ketanserin or broader antipsychotics such as risperidone under medical supervision. I cannot verify suitability, availability, or dosing for your case. For a concise overview of these options, see this article: ketanserine and risperidone as trip stoppers.
In short, prioritize environment, support, and reassurance first, and consider medication only with clinicians. Stay safe and get help if in doubt.