Verbascum Arctic Summer - Now a Giant!

I bought a teeny Verbascum Arctic Summer last year,  It grew to be about 2ft tall and produced a stunning lemon stem of flowers and soft downy leaves,  - its next to my Artichoke, and together they are amazing looking.  This year though,  its really settled and happy as its grown to be over 5ft tall  and still growing! 

How tall can they reach? Do I cut the flower stem once its bloomed?  the leaves at the bottom are also huge and the whole plant is taking over,  my once tall majestic foxgloves look like little daisies in comparison!