How to keep user decision workitem in user inbox untill particular condition is occured?

Hi,

I am working on Journal entry workflow .

i am using user decision step with Approve and Reject Button. but when user try to Reject particular document number i need to check condition period open or closed.

if period is closed then i need to display error message. is it possible to display error message ?

untill the period is open by adminstrator .user must cannot Reject Document number .

when period is open he can Reject . so untill that period is open i need to keep that user decision work item in user inbox only.

Please help how to acheive this requirement.

Thanks,

phani