How did your tenant get physical access to your DSL modem? Is it really a DSL modem or is it a combination modem/router? If your modem and router are two different pieces of equipment, then the direct connection of your shopkeeper tenant's router to your modem did indeed interfere with your other tenants' ability to pass traffic to the internet. If, on the other hand, you are served by a combination modem/router/WiFi access point device, then your problem is probably unrelated to your shopkeeper's having plugged into an available wired jack.
Do you have a WEP or WPA key protecting your WiFi? You might have a neighbor trying to crack your security in order to get free internet. One way of discovering your key is to evict your legitimate subscribers, forcing them to re-authenticate and giving the bad guy a chance to analyze that process.