More so, MsgPlus! is not a company. Messenger Plus! is the product of a single man, who we call Patchou.
He had quit his job to work full-time on MsgPlus! and in order to not starve, he added a nice sponsor (which does not harm in any way your computer).
To remove the sponsor, try the instructions in the link you were given.
There is no way it was installed if you refused. Make sure you're not confusing the adware with any other sponsor of another program.
Try installing MsgPlus! 3.40.0112 BETA, as some bits of the sponsor have been improved, and might be able to help you.
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