Is guiding someone to a tobacco shop considered an ongoing bad deed and an indication of evil? And does my sin increase every time that person buys cigarettes? And how can I erase the sin of that if it is an ongoing bad deed?

If you strongly suspect that the questioner intends to buy cigarettes, you must refrain from guiding him, as that constitutes aiding in sin. Allah Almighty says: (And do not cooperate in sin and aggression). And whoever calls to misguidance will bear a burden of sins similar to those who follow him. Erasing this sin is achieved through sincere repentance, which means regret and a firm resolve not to return to it. If you repent, Allah will wipe away your sin, even if the man continues to buy tobacco, because one who repents from a sin is like one who has no sin. The majority of scholars of Usul (Islamic legal theory) believe that repentance is valid even if the effect of the sin remains, provided the repentant person does everything within their power.