What should I do with my colleagues who do not pray or fast, despite my attempts to advise them and remind them of the seriousness of abandoning prayer and fasting?

Abandoning prayer and fasting are among the gravest of major sins. Many scholars have gone so far as to consider one who intentionally abandons prayer until its time has expired to be a disbeliever. Therefore, you should advise them and explain the gravity of their situation. If they repent, then praise be to Allah. But if they persist in their disobedience, then shun them if there is a benefit for them in doing so, and strive to supplicate for their guidance.