How can I delete all hidden buffers?

Try the following function:

function DeleteHiddenBuffers()
    let tpbl=[]
    call map(range(1, tabpagenr('$')), 'extend(tpbl, tabpagebuflist(v:val))')
    for buf in filter(range(1, bufnr('$')), 'bufexists(v:val) && index(tpbl, v:val)==-1')
        silent execute 'bwipeout' buf
    endfor
endfunction