Article 13 of the charter provides such mechanism to expel members, it is just matter of other members to agree on what is "violation and failing obligation", it is just the will nothing else.
SCO membership of a member State violating the provisions of this Charter and/or systematically failing to meet its obligations under international treaties and instruments, concluded in the framework of SCO, may be suspended by a decision of the Council of Heads of State adopted on the basis of a representation made by the Council of Ministers of Foreign Affairs. If this State goes on violating its obligations, the Council of Heads of State may take a decision to expel it from SCO as of the date fixed by the Council itself.
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It was a "mistake" to let in India in SCO, but it will be a even bigger mistake to expel India in the short term in high profile. It would only serve the interest of US for SCO to do so. The best approach for SCO is to keep India marginized in the orgnization. India can only do some small trick without openly breaking SCO's decision made by majority which India still has its name signed up to. If India choose to do more, then it is its own decision to leave, then it is time for the expulsion and nobody but India can be blamed. Just look at how Turkey works in NATO, no matter how many noises Turkey is still treated and acted as NATO by all parties inside and outside.
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