First of all find out whether the strike is legal or illegal. If it is illegal then organize domestic enquiry initiate severe action instead of issuing warning otherwise call the employees who participated in the strike ask them to submit an apology letter and then only allow them to work.
With out any complaint or commitment it is not correct to issue any warnings etc., if the employee is dare enough to go on strike and after issuing the warning letter they may challenge the management that the management is targeting or vindictive. Ask for a complaint from the concerned supervisor, supported by the Security and issue charge sheets, basing on their explanation issue warning letter ( if they accepts and begs for pardon ), if they are not repenting or accepting their mistake go for domestic enquiry, complete formalities and initiate action.
Regards - kamesh