European soccer's governing body said the six clubs had outstanding payments owed to other clubs, employees, social security or tax authorities.
The clubs were Astra Ploiesti (Romania), Metalurg Donetsk (Ukraine), Hajduk Split (Croatia), HSK Zrinjski (Bosnia), Skonto FC (Latvia) and Trabzonspor (Turkey).
UEFA said the decision was taken based on information supplied on June 30. Further information was requested from another 25 clubs.
Malaga, who should have qualified for this season's Europa League, have already been banned from European competition for a season because of overdue payments to rivals clubs and the Spanish tax authorities.
UEFA said the sanctions would remain in force until the clubs settled their payments in full.
UEFA is introducing new rules, dubbed financial fair play, to prevent clubs from spending more than the income they generate through gate receipts, sponsorship, television rights and commercial sales.
The rules were introduced because many clubs were running up huge debts and others were receiving huge injections from rich owners, distorting the transfer market and giving them what UEFA believed to be an unfair advantage.
From next season, clubs who do not comply will not be allowed to play in European competition.