Hello Ivan,
What I can suggest in your case is to use the OnClientClick property where you can call your javascript function and cancel the postback with return false:
OnClientClick="myfunction(); return false;"
Then once the spellcheck is finished, you can submit the form with Javascript.
I hope this helps. If you still experience problems, please open a support ticket and send us a small sample project which reproduces the problem - we will check it and do our best to help.
Greetings,
Georgi Tunev
the Telerik team