Before venturing into action, I would request for heart to heart talk with the employee. I remember the lesson I learnt way back in early 80's. We had recruited a telex operator for the marketing unit (No e-mail, scan, fax or sms/whatsup those days) who had to type messages running to several pages. The fellow was very good, very efficient and very well behaved. After about six months he started showing indifference and in a couple of months he became indisciplined, disrespectful and slightly abusive towards one and all. Conclusions were reached that he is to be thrown out. It was at this time the matter was referred to HR for taking action. When he was called he vented out his anger and frustration. Since he was in the department of international marketing there used to telex going/coming always and he was glued to his seat most of the time in a small cabin with only walls to stare. He often had to sit late also. He became so frustrated with his isolation and it found expression in the behaviour. There was a typist available who did not have enough work load and posting her in the telex department solved all the problems and frustration. Soon the office became a better place.
Though action could be taken as per Rules including disciplinary action, but I think an attempt should be made to understand the real issues and take corrective action.