How to make server side validations

Question

When user tries to submit form forcefully even after Validation message occurs then another message comes which is 'prevent this page from creating additional dialogs' and a checkbox after selecting checkbox script is disabled .My question is
1.  that how to prevent  this message
2. Or how to make server side validations which are created using  filtertextboxextender .
Shall we use regularexpressvalidator ?
3. Is there any alternative?
Thanks in advance