You cannot impose employees that they should not resign for particular time frame after increment.
However you may introduce increment reversal clause, which can be added in the increment letter, stating that in case of resignation within____months from the effective date of increment, Management reserves its right to reverse increment.
This is followed in IT companies.
Also please find out the root cause for their search even after increment immediately. Please conduct one to one session regularly, get their feedback, suggestions, thereby you can reduce the attrition rate.