The 20-year-old France international was fined and suspended until Monday for missing Thursday's training, but the ban has been extended indefinitely.
Dortmund met with Barcelona's representatives but the Spanish club did not meet their valuation.
The German club say that Dembele moving to Barca is "not currently likely".
Dortmund director of football Michael Zorc said: "Our focus now is on preparation for the Bundesliga opening match in Wolfsburg [on 19 August]. Dembele, of course, has the opportunity to complete an individual training session outside the group."
Dortmund signed Dembele on a five-year deal from Rennes in May 2016.

Cheshire Police released the statistic as they launched a campaign to urge male victims to report abuse.
Officers said they were raising the issue in the run-up to Valentine's Day, "when relationships are at the forefront of everyone's mind".
Information about help will be in GP surgeries, community centres, shops and sports centres.
Assistant Chief Constable Phil Thompson said: "Domestic abuse knows no boundaries and it can happen to anyone at any time regardless of gender, age and sexuality.
"Men in particular find it hard to speak out if they are being abused, but we want to reassure anyone who may be suffering at the hands of another, that domestic abuse is an issue that we take extremely seriously - whatever time of the year.
"Traditionally the emphasis has been on female victims of abuse but it is important to highlight the fact that many victims of domestic abuse are also men.
"By bringing this important issue to the front of people's minds we hope that it will encourage any men to have the confidence in contacting us if they want to end the cycle of abuse."

The Al-Qadi family saw their side go behind to Jamie Devitt's penalty when Tom Parkes was penalised for handball.
But Rory Gaffney equalised with a 12-yard volley after away goalkeeper Barry Roche's unconvincing attempts to deal with a Danny Leadbitter cross.
And substitute Billy Bodin's close-range header earned victory late on.
Rovers go sixth, five points adrift of the top three, while Morecambe are in 17th place, 11 points clear of the relegation zone.

The industry lobby group the British Retail Consortium and KPMG said sales had risen 5.6% since April 2016 - up from a fall of 1% in March.
Food spending accounted for most of the growth, but shoppers were more cautious about other items.
The BRC said consumer spending would slow as inflation climbed.
Chief executive Helen Dickinson OBE said: "The positive distortion from the timing of Easter was largely responsible for the month's growth and looking to the longer-term signs of a slowdown, the outlook isn't as rosy."
According to the research, sales of food and drink soared over the long Easter weekend boosting total spending.
Over the three months to April, total food sales increased by 3.6% - much faster than the 12-month average of 2% growth, it found.
However, quarterly sales of non-food items grew more slowly at 0.7%.
Exceptions included sales of children's clothes and toys and furniture, although researchers said these rises were seasonal.
According to official statistics, UK retail sales posted their biggest quarterly fall in seven years in March, as the prices of everyday goods continued to climb.
The value of the pound has slumped by around 14% against the dollar since the EU referendum last June, pushing up the price of imported goods. This threatens to derail a consumer spending boom that has underpinned economic growth since the referendum.
Ms Dickinson said: "Although today's figures do indicate that consumers are still willing to spend, with a cocktail of rising costs and slowing wage growth as the backdrop, conditions for consumers will get tougher."
Borussia Dortmund have suspended forward Ousmane Dembele "until further notice", three days after the club rejected a bid from Barcelona.
[NEXT_CONCEPT]
One in five victims of domestic violence in Cheshire is male, police have revealed.
[NEXT_CONCEPT]
Bristol Rovers celebrated their first match under their new Jordanian owners with a victory over Morecambe that takes them up to fifth in League Two.
[NEXT_CONCEPT]
Retail sales jumped in April thanks to higher spending over Easter, although tougher times may be ahead, a survey has found.