Questions & Answers
Do I have a ban if I've resigned before my visa was stamped in the passport?
I resigned during the probation period. My employment visa was not stamped on my passport, I did no medical and didn't receive Emirates ID.
Then, when I tried to get UAE tourist visa, the agency told me that I'm blacklisted and my visa rejected. My contract before was an unlimited contract.

Since you did not complete 6 months of probation, in all likelihood your employment can be terminated without warning.
Working on a visit visa: It is illegal for an individual to work without a valid employment contract/visa approved by the Ministry of Human Resources & Emiratisation ("Ministry of Labour") or the concerned Free Zone Authority.
You must have overstayed on a visit visa. If anyone overstays a visa, whether a tourist visa or after the termination of employment on the expiry of the grace period of 30 days, they are subject to daily fines and can also be deported under the Federal Law No. (6) of 1973 on Entry and Residence of Foreigners and under Federal Law No. (13) of 1996 Concerning Aliens Entry and Residence with a permanent/lifetime or temporary ban.