I can see two solutions here:
1. Deter students from borrowing or walking away with equipment
If safety is not a concern, you could require a shoe as a deposit. No student will get very far after class without realizing what they're missing. It will save you having to remind your students to return said items.
You could also take a tally of all loaned items and release the class only when all have been returned.
Another solution is for the replacement equipment to be clumsy, comical, or embarrassing. Giant calculators might get the job done, but they won't be inadvertently taken out of class and you might not see many repeat customers.
Unfortunately, any deterrent besides loudly pronouncing at the end of the class "please return anything you have borrowed!", or perhaps taking a tally, may be difficult to implement without giving a distinct "I don't trust you and am going to treat you all like children" impression.
2. Stop lending out equipment they are supposed to have.
A more cynical point of view is that university students should be intelligent, responsible, and well enough prepared to bring any equipment that is either listed as a course requirement to have, or is generally expected (e.g. a calculator, stationery).
Your students are probably less likely to prepare properly for class precisely because they know you have spare equipment. If they're supposed to have their own stuff, make it the students' own problem if they forgot. They will only learn if failure carries consequences. If you abolish your loaning policy, I suspect the fraction of students coming properly prepared will quickly, and significantly, improve.