Not in a normally supported manner no*.
You can actually remove the timesheet by deleting the time period (and then recreating it), but pretty sure this is not going to affect ActualsThru at all. (And obviously this would delete any other timesheets for that period for other users).
Lots of things you can do before this problem arises to prevent it (not allow future timesheet posting at system level, not opening future timeperiods that far in advance, catching the "error" in your approval process (somehow), instilling the importance of approval of timesheets if you have a manual process etc), but once it is posted, its a done deal.
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* so perhaps a conversation with support about a direct database update is your way forward.