When situations like this arise, it is imperative that you address the issue with your landlord in writing (date it and keep a copy). You may want to pursue the issue directly with the owner. A positive relationship with your landlord is worth is weight in gold, so be open and honest and propose a solution. If you are accusing other tenants of drug activity, then have evidence to back up that allegation. The landlord can evict tenants for illegal activity. Check this link for some good information: http://www.dca.ca.gov/publications/landlordbook/resolve-problems.shtml
You can bring a nuisance claim. You would need strong evidence about what they were doing; saying that they were dealers and gangsters would not be enough. You should probably see an attorney, but you can ultimately bring an action in small claims court. If enough neighbors do that, your landlord may get the message.