Peter Symonds College in Winchester has decided to close what it sees as a loophole by telling students who enrol for vocational courses that they risk losing a hefty subsidy paid by the Learning and Skills Council.
The LSC pays 72.5 per cent of the cost of courses such as beauty therapy, childcare, IT and modern languages. A 30-week French course costs learners pound;110, compared with pound;400 without any subsidy.
Neil Hopkins, principal of Peter Symonds, said the college had started to crackdown after a group of adults enrolled for an A-level in environmental science without any intention of taking the final exam.
If learners do not complete a qualification, the LSC withdraws the subsidy and the college bears the extra cost.
"People sometimes misunderstand how expensive adult education really is," he said.
From September, adults will be asked to state that they intend to complete the qualification and promise to complete forms to comply with LSC rules.
But Mr Hopkins admitted that the college was powerless to stop students dropping out or changing their mind at the end of a course and refusing to sit an exam.
Alan Tuckett, director of the National Institute of Adult Continuing Education, said colleges were having to make difficult judgements as money was tight, but added that some learners are put off by exams. "Higher fees are not a solution to the funding pressures in the short run," he said.