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.
Borussia Dortmund have suspended forward Ousmane Dembele "until further notice", three days after the club rejected a bid from Barcelona.