I agree with Yacine, for the employer its just expenses and waste of time hiring u and leaving u in a week..lets be fair here, he spent for ur tickets, visa, medical application, accomodation etc.
Anyhow, theres usually a clause in contract which states u can resign by serving a notice period of 1 week during probation. just resign and leave.
Regarding your other post, you cant come back on the same visa as its a single entry visa and ur employer has to again apply for a visa if he wants you back.