It seems that your lease does not clearly specify the limits of use of the part of the property you are describing. If it does, then this would give you more leverage to make the tenants stop using that area. If the lease is vague or does not mention what areas of the property tenants do/do not have access to, then the best you can do is take the position that there was a verbal understanding about these limits when the tenancy began. Clearly, this is not ideal, because if you have to take the matter further by serving an N5 notice to terminate due to the tenant interfering with your enjoyment or that of another tenant on the property, then your position is weaker than it should be. Clear contract terms are vitally important.