You can file for divorce if your spouse has undergone conversion to another religion and has ceased to be a Hindu.
File a Case
It is very important to note that since your spouse has converted to another religion it does not automatically end your marriage to them. You must still file for divorce.
Even if your spouse has converted to another religion the divorce proceedings will happen under the Hindu law and not the religion your spouse converted to. This is because your marriage to your spouse happened under Hindu law.
Marrying before Filing for a Divorce
Till the divorce has been finalized by the Court your marriage will continue to exist with your spouse. Your spouse cannot marry before this and such marriage will not be a valid marriage in law.
If your spouse marries again without getting a divorce from you then you can charge them with the offence of bigamy which is punishable with up to 7 years jail time and a fine.