The Mexico international joined the Bundesliga side in the summer form Manchester United and has been a revelation this season, scoring 19 goals in 25 games in all competitions.
As a result of his fine form, Arsenal have been linked with a move for the 27-year-old, but Voller maintains the club will not be persuaded to sell.
"We are a football club and it is important that we are successful," he told Sky Sport News.
"Chicharito has played a big part in our success so far, through his style of play, through his goals.
"That's why we brought him here. In the first half of the season, everything has worked out fine.
"It's easy to reject offers when you are in a position in which you don't need to sell. We are in such a position. We will keep the players we want to help us achieve our goals."