how to stop an integrated washing machine from moving...??1

hi out there... can someone give me some tips to stop a built under washing machine from moving during a spin cycle... after fitting the kitchen i tested the appliance and it didnt move after the cylce had finished but my customer insists it moves enough for the integrated door to be out of alignment...

i am sure its level (in all directions) and as far as I'm concerned i've done everyting as i would normally do.... any tips would be v useful and i'd be grateful....

thanks