Can I cut lupin foliage back after flowering?

You should cut out spent flower spikes, but is there any particular reason why you would wish to remove foliage?

Just pull the head,s of they sould flower again this year.Watch out for greenfly though they attrached them like mad.

Once the flower heads have turned to spikes of seed pods chop them off and you lupins will flower again , and quite often if you have strong plants you can repeat the process, after the first flowers the subsequent flowers will be smaller and as Milligan says do watch out for greenfly.In the Autumn cut them down to soil level, they are hardy perennials.