1. No case lies against refusal of marriage , more so, when the party is other than the couple.
2. Refusal of marriage on the ground of caste is hence not liable bring prosecution.
3. However f someone from higher caste makes derogatory remark on caste of the person of SC OR ST category then criminal case can be filed against him.
So in your case solely for refusal of marriage on caste you can file case against them.