>>> Can the association claim members personal items as their own just because they are used on a regular basis by the club?
Definitely not. Title to those items remains with their original owners unless they've
(a) sold them to the association ; or
(b) given them to the association as unconditional gifts ; or
(c) legally 'abandoned' the goods.
>>> If the members decided to leave would they be entitled to a rebate of unused club fees?
A well-written constitution should cover such matters. In the absence of any relevant provisions within the association's constitution though, the answer is almost certainly 'No'. (The constitution might not even recognise the idea of members 'leaving'; it might be that, even though they no longer take part in any of the association's activities, they're still regarded as members until the end of the membership year that they've paid for).