Getting rid of pot smell/ evicting tenants

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Hi everyone,

Just came across what it could be a good deal in a south side suburb of Chicago. Owner is motivated and numbers work pretty well. However, current tenants smoke pot pretty consistently. The whole house just smelled like pot and If I decide to buy the place, I'd decide to just get rid of them. They pay rent month to month and there is no contract. 

What are the rules for evicting tenants assuming there is no contract in place? and how hard is it to get rid of the pot smell in a house?